diff --git a/Day12.cs b/Day12.cs
index 6ebe4b8..c497f96 100644
--- a/Day12.cs
+++ b/Day12.cs
@@ -1,4 +1,4 @@
-#define Day12
+// #define Day12
 #if Day12
 using System.Collections.Immutable;
 using AoC2021;
diff --git a/Day13.cs b/Day13.cs
new file mode 100644
index 0000000..7005b1b
--- /dev/null
+++ b/Day13.cs
@@ -0,0 +1,52 @@
+#define Day13
+using System.Text.RegularExpressions;
+using System.Threading.Channels;
+#if Day13
+var text = File.ReadAllText("day13/input").Split("\n\n");
+var points = text[0].Split("\n").Select(l => l.Split(","))
+    .Select(p => new Point(int.Parse(p[0]), int.Parse(p[1]))).ToHashSet();
+
+var reg = new Regex(@"fold along (?<c>x|y)=(?<p>\d+)");
+var folds = reg.Matches(text[1]).Select(m => new Fold(m.Groups["c"].Value[0], int.Parse(m.Groups["p"].Value))).ToList();
+
+foreach (var curFold in folds) {
+    HashSet<Point> next = new HashSet<Point>();
+    if (curFold.c == 'x') {
+        foreach(var p in points) {
+            if (p.x < curFold.p) {
+                next.Add(p);
+            } else if (p.x > curFold.p) {
+                next.Add(new Point(curFold.p - (p.x - curFold.p), p.y));
+            }
+        }
+    } else {
+        foreach(var p in points) {
+            if (p.y < curFold.p) {
+                next.Add(p);
+            } else if (p.y > curFold.p){
+                next.Add(new Point(p.x, curFold.p - (p.y - curFold.p)));
+            }
+        }
+    }
+    points = next;
+    Console.WriteLine(points.Count);
+}
+
+var maxX = points.Max(p => p.x);
+var maxY = points.Max(p => p.y);
+
+for (int y = 0; y <= maxY; y++) {
+    for (int x = 0; x <= maxX; x++) {
+        if (points.Contains(new Point(x, y))) {
+            Console.Write("#");
+        } else {
+            Console.Write(".");
+        }
+    }
+    Console.WriteLine();
+}
+
+
+record Point(int x, int y);
+record Fold(char c, int p);
+#endif
\ No newline at end of file
diff --git a/day13/input b/day13/input
new file mode 100644
index 0000000..f8f846c
--- /dev/null
+++ b/day13/input
@@ -0,0 +1,849 @@
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+
+fold along x=655
+fold along y=447
+fold along x=327
+fold along y=223
+fold along x=163
+fold along y=111
+fold along x=81
+fold along y=55
+fold along x=40
+fold along y=27
+fold along y=13
+fold along y=6