diff --git a/2021/day21.py b/2021/day21.py
new file mode 100644
index 0000000..e6cf861
--- /dev/null
+++ b/2021/day21.py
@@ -0,0 +1,48 @@
+from functools import cache
+
+with open("input21.txt") as f:
+    lines = [line.rstrip() for line in f]
+ipos = (int(lines[0].rsplit()[-1]), int(lines[1].rsplit()[-1]))
+
+pos = list(ipos)
+scores = [0, 0]
+num_rolls = 0
+turn = 0
+while scores[0] < 1000 and scores[1] < 1000:
+    roll = sum(range(num_rolls + 1, num_rolls + 4))
+    num_rolls += 3
+    pos[turn] = pos[turn] + roll
+    pos[turn] = pos[turn] - ((pos[turn] - 1) // 10) * 10
+    scores[turn] += pos[turn]
+    turn = (turn + 1) % 2
+print(min(scores) * num_rolls)
+
+rolls_prob = [
+    a + b + c for a in range(1, 4) for b in range(1, 4) for c in range(1, 4)
+]
+
+
+@cache
+def play(pos, scores, turn):
+    if scores[0] >= 21:
+        return 1, 0
+    if scores[1] >= 21:
+        return 0, 1
+
+    w1, w2 = 0, 0
+    nturn = (turn + 1) % 2
+    for r in rolls_prob:
+        p = pos[turn] + r
+        p = p - ((p - 1) // 10) * 10
+        if turn == 0:
+            npos = p, pos[1]
+            nscores = scores[0] + p, scores[1]
+        else:
+            npos = pos[0], p
+            nscores = scores[0], scores[1] + p
+        res1, res2 = play(npos, nscores, nturn)
+        w1, w2 = w1 + res1, w2 + res2
+    return w1, w2
+
+
+print(max(play(ipos, (0, 0), 0)))