diff --git a/2021/d25/ex1/ex1.py b/2021/d25/ex1/ex1.py
new file mode 100755
index 0000000..c785a12
--- /dev/null
+++ b/2021/d25/ex1/ex1.py
@@ -0,0 +1,88 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from typing import List, NamedTuple, Set
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+class Map(NamedTuple):
+    east: Set[Point]
+    south: Set[Point]
+    dimensions: Point
+
+
+def solve(input: List[str]) -> int:
+    def parse() -> Map:
+        east, south = set(), set()
+
+        for x, line in enumerate(input):
+            for y, c in enumerate(line):
+                if c == ".":
+                    continue
+                if c == "v":
+                    south.add(Point(x, y))
+                if c == ">":
+                    east.add(Point(x, y))
+
+        return Map(east, south, Point(len(input), len(input[0])))
+
+    def step(sea_cucumbers: Map) -> Map:
+        def move_east(p: Point) -> Point:
+            return Point(p.x, (p.y + 1) % sea_cucumbers.dimensions.y)
+
+        def move_south(p: Point) -> Point:
+            return Point((p.x + 1) % sea_cucumbers.dimensions.x, p.y)
+
+        east, south = set(), set()
+
+        for old_p in sea_cucumbers.east:
+            p = move_east(old_p)
+            if p in sea_cucumbers.east or p in sea_cucumbers.south:
+                east.add(old_p)
+                continue
+            east.add(p)
+
+        for old_p in sea_cucumbers.south:
+            p = move_south(old_p)
+            if p in east or p in sea_cucumbers.south:
+                south.add(old_p)
+                continue
+            south.add(p)
+
+        return Map(east, south, sea_cucumbers.dimensions)
+
+    def debug(map: Map) -> None:
+        for x in range(map.dimensions.x):
+            print(
+                "".join(
+                    "v"
+                    if Point(x, y) in map.south
+                    else ">"
+                    if Point(x, y) in map.east
+                    else "."
+                    for y in range(map.dimensions.y)
+                )
+            )
+
+    sea_cucumbers = parse()
+    for i in itertools.count(1):
+        if (new_map := step(sea_cucumbers)) == sea_cucumbers:
+            return i
+        print(i)
+        sea_cucumbers = new_map
+
+    assert False  # Sanity check
+
+
+def main() -> None:
+    input = [line.rstrip("\n") for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()