diff --git a/2021/day5.py b/2021/day5.py
new file mode 100644
index 0000000..f3bb9bf
--- /dev/null
+++ b/2021/day5.py
@@ -0,0 +1,43 @@
+import sys
+from collections import defaultdict
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+    coords = [
+        tuple(map(
+            lambda s: tuple(map(int, s.split(","))),
+            line.split(" -> ")
+        ))
+        for line in lines
+    ]
+
+    # Part 1
+    data = defaultdict(int)
+    for (x1, y1), (x2, y2) in coords:
+        if x1 == x2:
+            ymin, ymax = min(y1, y2), max(y1, y2)
+            for y in range(ymin, ymax + 1):
+                data[(x1, y)] += 1
+        elif y1 == y2:
+            xmin, xmax = min(x1, x2), max(x1, x2)
+            for x in range(xmin, xmax + 1):
+                data[(x, y1)] += 1
+    print(sum(v > 1 for v in data.values()))
+
+    # Part 2 - reuse previous data and just consider diagonals
+    for (x1, y1), (x2, y2) in coords:
+        if x1 != x2 and y1 != y2:
+            xofs = 1 if x1 < x2 else -1
+            yofs = 1 if y1 < y2 else -1
+            while True:
+                data[(x1, y1)] += 1
+                if x1 == x2:
+                    break
+                x1 += xofs
+                y1 += yofs
+    print(sum(v > 1 for v in data.values()))
+
+
+if __name__ == "__main__":
+    main()