diff --git a/advent-of-code-2021.csproj b/advent-of-code-2021.csproj
index 8c40388..da056ec 100644
--- a/advent-of-code-2021.csproj
+++ b/advent-of-code-2021.csproj
@@ -72,6 +72,9 @@
 	  <None Update="inputs\18.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\20.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	</ItemGroup>
 
 </Project>
diff --git a/inputs/20.txt b/inputs/20.txt
new file mode 100644
index 0000000..2e2b2bd
--- /dev/null
+++ b/inputs/20.txt
@@ -0,0 +1,102 @@
+###.#####..###..###.....##.##.#..#.#######..#...####.....#.#.###..##.#....####.#...##.#.#..##.###..######.#.#.#.##..#...##..###....####.##.##..#.#..##...##.#..#..#...#..#....###.####.######..#..#..#..#..........#.####...#..####.##.....#.#..#..#..####.##..#...####..#....#...##.#####...##.#.##.#..#....###....###..#...#....###.#....#..##....#..#..#..#..#.......###..##..##..####...###..#.....######.#.#.#.####.####.#####.....#..#...#.#.###...#...#.##.##.....#....#..##.###..#.##.#.##..#.#......####.#....#..#.#...
+
+...#.####...#...#..#####..#.#.##........###..#.....##..####.###.#####......#.##.#..###.#####..#..###
+#...#.#....####.##.#...###..##...#.###...###...#..#.##.#.###..#.#.##..#.####.#..#..#.####.##.#..#...
+....#.##..##.#####.#...#########..#.#.####....#..#.#..##.##.###.#.####.##.###.##...#.######..#.#.#..
+..#.##.###.##..#.###..#.#..#.#..###..........#..#..###.#.......###.##.#.#..#####.##.###.###.###..#.#
+##...#...#.###.#####..#.#####.#..##.#.##.#..#.##.#..#...##...####.#.#..#....#########.#.##...####.##
+###.##...#.###...#.####...#.#.##..##......##..#.###..#####..##...##.##..##..##.##...#.#.#..#.#..##..
+.#.#####....#...#...#..##..##..#....#..##..##.##..#..#.##..#.#.###..##..###..#...#....##.##...##.###
+.##.#..#####.##.#.#..#..#.#####.....##.####.....#######.#.####.####..#.#.###.#....###.#...##..#.#.##
+#.#.#..###..###.#.###..###...##.##.#....##.#.###..######....##.##......##.#..#..######.###.##..###..
+#..######..##..####..#.#....#...#.#.###....###....#.#..##.###.#..#.#..#.##.#....#....#....##..##....
+.##....#...###...##..#...#....##.####..#.##..#.....#....####.#..#...##.#..###.#.#.####....##.#...###
+##.#######...###...........##.###..##..######..###..#.##.###..#.##...#.####...#....###.#..#.##.###.#
+.#..#..#..#...###....#.#..#.####.#.#....###...#.#...#..##.##..#...###.#.#.....#.#.#.###....###.#...#
+###..##.####..####.#.##..#..#.######.#.#####....#..#.#......#........#.#..##.#.####...###.#..#.##..#
+.##.#.####.#.##.###..#..#....#####.#.....#..#....###.....#.#..######..#..#....#.##....####.###.#.##.
+.#..#.....##....#.#.#...######...####.#####..#.#.#.##.#..#####.#.#.#.##..#..##.##.......##..#.#.##..
+###..##...####....##.#...#..##.##.###.#......##...##.....#.#.#.....#.###.##..####..##.####.#.###.###
+..#...#...####.....####.#.#.#..##..##..#....##.##.###.#.#####.#..##.##.##...##..#.###..#.#...##....#
+.#.##.#..####.#.#..##...#......###.#..##.##.........####.###.#..##.####..#.#.#.#.###...#.#...##.##.#
+##.#..#...##.##.##.#...###.....##.#####.#.#.##...#.#.###..#.##..#.##...#...#..#..#..####..##..###..#
+...#####.##..##.###.#.#..###......#..##..#..#######.#.#.......##.#.#.###.##.#.#.####...##.#..#....##
+#.#...########.##.#.#..####.###.####.##.#####..##.###.#.#.###...#....##..#..###.#.....#.###.##.....#
+.#.##.##.##...#####..##..##.###.######.......#.#.#....##.###..###..##.###.#.#..#..#..####...#..#.#..
+##.###.###....########....#.#...###..#.##.#....###..#.#.....###..#.#..#.##.#.......######.#.#.##....
+##.#.##..#....#.#..##.#.###.##...####.....#...#.#######....#..#.#.#.#.#..#.#.#.##.####..#..#...###..
+.#.######.##.#..#..###.#.###..#.#.#..#...#..##.#######.#...#...#..####.#....###...#.###.##...##....#
+.#.##.#.#.#..#..#.####.##..####..#..###.#.##..#..##........#.#.#.#########.#..####..#..#.##.##....#.
+..###.#..##.##.##...###.##...##.##.##.#.##.###.#####.##..#####.##..######..####.##.#.##..#.....##...
+##..####..#.##....##.#..##..#.#.#..#.##.....#..#....#.#.###.##.##.####.##....#.#.##......#....#.....
+..#.......#.#....##.#.#.###..####..###..#.#.####.....#...#..##.#..#..#...######...........##..##...#
+...#.#...#....##.#....##.##.##.#.#..#.##.#.###.#.#.#.#.#######.#.##.##..#..##.#.....#..##..#.##.#...
+.#.#..#..#..##..#....#.#..#....##....#.###.###...#..#...####.###.....#.##.#..###.....##..#.#.##..#..
+.#....##..##..#..#...##.##.#.##..#.##.#.#...#####..#....##...##...#...####.###.#..#####.#..##..#..##
+###.#####.#.#...#.#.#######.#.....#####.#...#..#.##.###...#..#..#.#..##..#.#...###.##.#..##..###.#.#
+.#####.###.#######..#.#......#.##.#.##.#.#...##....##.#.#.#...#..#....##...#..#...#...###....#..#.##
+###...##.##.##.##..####.##.######.#.######.##..#.#...##...#....####...#..##....#..#..#####....#.#..#
+#...###.#.#.###...####...##.#.#..###..#######.##.####..#.####..##.#.##.###.######.........###.#.#.##
+####.###.#.#.#.####...#..#..######.#..####..##.#.###...#.#.#.##....#..#.#.##.#..#....#...##.##..###.
+#...#..##.#...#..##..##.####.#..###.##.#...#.#.#...##.##.#.#....##...###.#...#...#..#.#.#.##.##....#
+...#...###...#..######.#.####.#.#..####.....#.####..###.#.######..##.......##.#...#..##..#.###..###.
+.##.##.##..#..###.#...##..#.#.#..##..###.#.###.#.#.##.....#.#.###....#.#..##.##..##.###.###.####..##
+###.###....####..#..#.##.#.#..#.####..##.##.#..##..##.#.#.#.##..###...#.#.##.#..##.##..#.#..##.##.##
+##...####.##.##...#.###...##..#.##..#.##...####..###.....#.##.#.##.#.#....###.........#.#.#..###.###
+.#..##.###..##.#..#.#####......#..#####.#.###...###.#....##...#####..###.####.###..##.#.#...#...#.#.
+..#...#..####....#..#......#.##...######.##.###.##.##.##.#......#####.#..##.#...##...###..###...####
+..#..###.####.####.####.###..##...#########....######..#....##.###...#.#........##...##...#..##..###
+.##..#......###.#...##.#..#.#..####.###.#.###..##.##.#...###.#.....##..#..#.#.#..#......#.####.##.#.
+###..##.#.#.###..#....#.##.###.........#.##...#.###...##...#.##....#.#.#.#..###....###.###..#.##...#
+###.#..##.#...####..##..##.#...#.##..#..##.####.#..#.#.#..#.##....#..#......###....####......#.#...#
+.#######...###...###.##.#.#..#.....##...#...#.####..##.#.##..#.#...#..###........##..##.#.##..#.....
+..####.#####...##..##.###.##.#..#..##...#.#....##.#.#...##...###...##.#....###.#.##....#.....#.#.#.#
+...#.......#..#.###.....##.#.......#...#..##....#####.##.###.....##....##.#....#.#.##.#.#.#..####.#.
+.#..#.#..#...#.#.#..#..#..###..#####..###.#.#.####.##..#.#....####.##...#...#....##..##.##...###.#.#
+.#.##..#######..#.......#.###..#.#.###.#..##.##......##.####.##.###......#.###.#.#...#...##.....##..
+#..####.....#..#...###.#.###..#...#.###.#...###...##.##....##.##..#.###.###.###.#.##...####.#.##.##.
+.###.....#####.##.#...####..#.##..###########..#...######.......###.......####.###....#.###.....###.
+.#.####..#..#.##...#.#######.#.###....#.###.#..#.#.#..##.#.#.#..#.#.#.....#..#..###..##..#....####.#
+.#####..#..#....#.###.#.###...#.###..#.#.##.#....#.####...##....##.#####.####.##..#.#.#####..####.##
+#...##....###..###...##...#..#......#..#.##..####..##.#.#......#..##.#..##..###.##..###...#..####.##
+....####.####..........#...#..####..#.....#....##.####.#.##..#..#....##...#.#...##.#.#..##.##.##...#
+.....#.#...#...#.###..#..#.#....#....#..#.###.##.#.##.##..#.#.....#.###....####.#....#.....#.#..#.##
+#...#.##..#.#..##.#.....#.#.#.#..#.####..#...#.#.#...##.....#.#####..##..##..###.##.#....#.....#....
+.####.##..##.......##.####..##......##...#.#.###..#.....##.#..#.##.#.#.#.###.......###.##.#.........
+#.###.#.#.###.#.#.#..##.#..#.######..#...#....#..###.##.#...#.#.#.#.####....##..###.###....#.#.###..
+.#.#..#.....####.#####..##..####....###....#....##.##......##..#.#.#...#.#...##.##.##.######.#.#.###
+###..#.#..###..#..##..###.....#####.#.#...#.##......#.#.#........#.#...#.....##..##...#.#....#.#..#.
+##....#####...#.#..###.####...#...##....##..#.#...#...##.##...##........#.#....#####..#.....####..#.
+.#..#.#####.#...#.#.#..###.##...##...##..##......##.#.###...#.#..#.#####.#.#..##...##..##...###.....
+.##..#..##..#.###..####.....##.#..##.#.#...##.#########..#..###..#.##.#.#..#.#.#.###.###..####..##.#
+##..#..#...#..##...#.#.##...#....##..#.#.##.....##.#####.##..##....#.#......#.###.....#.###.....#...
+#.#.#..#..##...#.##.##.###.###......#...###....#.#..##..#.##..#..#.##.#####...#...###.#####..#...###
+...#..##.######.#..#..##..#..##..#....#.#.#..#...#.##....##.#.##.##....###.#.######..#..###...##.#..
+###...######..#.##.##.#.#.###.#.#.#...###.##...##.#.##.#.#.#..#....####...###.#..##.##.#.##..#..#..#
+#.##..###.######..#..#.##.##.#.##..##..#.##..#.##..##.######.#.#....#.#...#...##..#.#.##.#.#...#.#.#
+..##.#.#...###.##.....#.#.#.#.#####..###.#..##..##.#.#..##.#.###..#..#.##....#....##...#######.#.#.#
+.#.###.....#..#.#.....#..#.....#....#....##..###.##.##.#.#####....#.####.....##....#.####.#.##...##.
+#.####.##..#..#....###.#.......####..##.##..##...#...#..##..##.#...#.#..#..#.##.##.#..########..####
+.###.#.#..#######...#.#.###.#.#.##.....#.#..##.#..##..#.#....##..##..##..##.#.#.#..#.#..#.#...#..#..
+###.#.#.#....#.#..#..###..#..#.####.#.#....#.#####..#.####.#.##.#######.##...###.#...#..#.####..###.
+.#.....###.##.##..#..###...####....####...#..#...###..##...##.###.###..#.##.#.#.####.....##.#.......
+.#..##.#.#...##.#..........##.####.#.###..##....#.....#.####........#.#.#.##.##.##.#...##..#..###...
+#.#.###.###..#.##..#.##..##...#..#.......####...#.#..#...##.#.##...#..##.###.####.#......#.......###
+..###..#..####.#.#...#..###..#.#.#...#####..#..#####..#.......#.#..#.###..#..#.##.######.###.#.#..#.
+#.###.##.#..#...##.#.##.##..###.#.#.#..#.....#..##..#...#.#####.#.#.#.........#.#####.#..#####...#.#
+#..#.#...#####.#.##.##.#..##..###.##.#...##..#.##.#..#.###.#.##.#.#..##.###.###..#.#..#..#...##.##.#
+#...##.###..#.#...##.##...##..#..#.#.##.#.#..####..#####...##.##.####...#.###.#.##...##...#.##.##.##
+#.##..###..###.#...#.....#...##...#..##.##.#..##...###.#.....#.##.......#####.#####....#.###.#######
+..#...##.#..####...###..#.##...#.##.#.....##..##...#.#..#.#.#.##.......#...#.###..#....##.######....
+###.#..###..##.#........##.#.#.#..#....##.#...#....#.###..##.##.##.###..####.#####.....##...#..#.##.
+.####.##.....##............##.#...##.###...#.##..########.#..###......##..##.##..##..#.###....#.##..
+##.##.##.#..#...########.####.####.#.#.###..#...#.#.###..##.#.#.####.#..#.#.....#..#..##.#..##.##...
+##.##.#..#.....##.#####..#........####.#.#.##....#..#..#....###....#.......#.###.##.##...#..###..###
+##...#..#...#.#...##.#..###..###....#.#.......#...##.....#...#..#.#..###.##.##..###..#.#########.##.
+.#######...##.#..##.#####.....#....###.####.#.#.#..##.#.#..##...##.##.#.###.#.#....###.##.#.###.#..#
+..##.##..##.##.#.#..#.#..#.##..####..###..#######..#..#.#.#...###...#.###.#...#.###.##....##..#.###.
+#.......#...#.##...#.#...##....#.#....##.#.###.#.#...#...#######...###.#.#.##....###.#.##.#...##.##.
+##.##.#..##....#.######..##...#.##.#...#.#.####..####.##...####.#.#.#.##.#....###..##.#######..#..#.
+.....##....#.##.##.#.##....#.#.##.##......####.##..##.#....#..#.##.###.#..#....##.#..#.#.#.#.##...#.
+.#.#..#...#..#..#.....#####...#####..##.#.##.#.#....##..##............#.##...######..#.###..#...#..#
+#.#####.##.#.#.....#...#..#.#.#....#....#..##.#..#..#....####.#...#.##.####.#.##.#...#.#.#.#..#...##
\ No newline at end of file
diff --git a/src/20.cs b/src/20.cs
new file mode 100644
index 0000000..ad95c30
--- /dev/null
+++ b/src/20.cs
@@ -0,0 +1,114 @@
+namespace aoc2021;
+
+internal class Day20 : Day
+{
+    internal override void Go()
+    {
+        var lines = Util.ReadAllLines("inputs/20.txt");
+        string key = lines.ElementAt(0);
+        var grid = new List<List<byte>>(lines.ElementAt(2).Length);
+        for (int i = 2; i < lines.Count(); i++)
+        {
+            var line = lines.ElementAt(i);
+            var row = new List<byte>(line.Length);
+            grid.Add(row);
+            line.ForEach(ch => row.Add(ch switch
+            {
+                '#' => 1,
+                _ => 0,
+            }));
+        }
+
+        Part1(grid, key);
+        Part2(grid, key);
+    }
+
+    private static void Part1(List<List<byte>> grid, string key)
+    {
+        int numLit = grid.Sum(row => row.Count(col => col == 1));
+
+        using var t = new Timer();
+
+        (grid, numLit) = Enhance(grid, key);
+        (grid, numLit) = Enhance(grid, key);
+
+        t.Stop();
+
+        Logger.Log($"<+black>> part1: #lit=<+white>{numLit}<r>");
+    }
+
+    private static void Part2(List<List<byte>> grid, string key)
+    {
+        using var t = new Timer();
+
+        int numLit = 0;
+        for (int i = 0; i < 50; i++)
+        {
+            (grid, numLit) = Enhance(grid, key);
+        }
+
+        t.Stop();
+        Logger.Log($"<+black>> part2: #lit=<+white>{numLit}<r>");
+    }
+
+    private static bool IsValidIdx(int len, int idx) => idx >= 0 && idx < len;
+
+    private static (List<List<byte>> grid, int numLit) Enhance(List<List<byte>> grid, string key)
+    {
+        var newSize = grid.Count + 2;
+        var emptyRow = Enumerable.Range(0, newSize).Select(_ => (byte)0);
+        var outImg = new List<List<byte>>(newSize);
+        for (int rowIdx = 0; rowIdx < newSize; rowIdx++)
+        {
+            outImg.Add(new List<byte>(emptyRow));
+        }
+
+        var def = key[0] == '#' && grid.Count % 4 != 0 ? '1' : '0';
+        int numLit = 0;
+
+        for (int row = - 1; row < grid.Count + 1; row++)
+        {
+            for (int col = - 1; col < grid.Count + 1; col++)
+            {
+                var num = Enumerable.Range(0, 9).Select(_ => def).ToArray();
+                if (IsValidIdx(grid.Count, row - 1))
+                {
+                    if (IsValidIdx(grid.Count, col - 1))
+                        num[0] = (char)(grid[row - 1][col - 1] + '0');
+                    if (IsValidIdx(grid.Count, col))
+                        num[1] = (char)(grid[row - 1][col] + '0');
+                    if (IsValidIdx(grid.Count, col + 1))
+                        num[2] = (char)(grid[row - 1][col + 1] + '0');
+                }
+                if (IsValidIdx(grid.Count, row))
+                {
+                    if (IsValidIdx(grid.Count, col - 1))
+                        num[3] = (char)(grid[row][col - 1] + '0');
+                    if (IsValidIdx(grid.Count, col))
+                        num[4] = (char)(grid[row][col] + '0');
+                    if (IsValidIdx(grid.Count, col + 1))
+                        num[5] = (char)(grid[row][col + 1] + '0');
+                }
+                if (IsValidIdx(grid.Count, row + 1))
+                {
+                    if (IsValidIdx(grid.Count, col - 1))
+                        num[6] = (char)(grid[row + 1][col - 1] + '0');
+                    if (IsValidIdx(grid.Count, col))
+                        num[7] = (char)(grid[row + 1][col] + '0');
+                    if (IsValidIdx(grid.Count, col + 1))
+                        num[8] = (char)(grid[row + 1][col + 1] + '0');
+                }
+
+                var keyIdx = Convert.ToInt32(new string(num), 2);
+                var ch = (byte)(key[keyIdx] == '#' ? 1 : 0);
+                outImg[row + 1][col + 1] = ch;
+                if (ch == 1)
+                {
+                    numLit++;
+                }
+            }
+        }
+
+        return (outImg, numLit);
+    }
+}
diff --git a/src/main.cs b/src/main.cs
index 3c43b1a..e82507d 100644
--- a/src/main.cs
+++ b/src/main.cs
@@ -36,7 +36,8 @@ else
         "15" => new Day15(),
         "16" => new Day16(),
         "17"=> new Day17(),
-        _ => new Day18(),
+        "18" => new Day18(),
+        _ => new Day20(),
     };
     day.Go();
 }