diff --git a/2021/day4.py b/2021/day4.py
new file mode 100644
index 0000000..b833364
--- /dev/null
+++ b/2021/day4.py
@@ -0,0 +1,77 @@
+import sys
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+    pack = list(map(int, lines[0].split(",")))
+    boards = []
+    for i in range(2, len(lines), 6):
+        boards.append([
+            list(map(int, filter(bool, line.split())))
+            for line in lines[i:i+5]
+        ])
+
+    # Part 1
+    find_winner(boards, pack)
+
+    # Part 2
+    not_won_yet = [1] * len(boards)
+    iterpack = iter(pack)  # just so we can avoid looping twice
+    for n in iterpack:
+        for b, board in enumerate(boards):
+            board_marked = False
+            for x, row in enumerate(board):
+                for y, num in enumerate(row):
+                    if num == n:
+                        board[x][y] = -1
+                        board_marked = True
+                        break
+                if board_marked:
+                    break
+            if get_unmarked_sum_if_winning(board) is not None:
+                not_won_yet[b] = 0
+        if sum(not_won_yet) == 1:
+            print("We have a loser...")
+            ib = not_won_yet.index(1)
+            find_winner([boards[ib]], iterpack)
+            break
+
+
+def find_winner(boards, pack):
+    won = False
+    for n in pack:
+        for board in boards:
+            board_marked = False
+            for x, row in enumerate(board):
+                for y, num in enumerate(row):
+                    if num == n:
+                        board[x][y] = -1
+                        board_marked = True
+                        break
+                if board_marked:
+                    break
+            if unmarked_sum := get_unmarked_sum_if_winning(board):
+                score = unmarked_sum * n
+                print("We have a winner!", score)
+                won = True
+                break
+        if won:
+            break
+
+
+def get_unmarked_sum_if_winning(board):
+    for row in board:
+        if row == [-1, -1, -1, -1, -1]:
+            return get_unmarked_sum(board)
+    for col in range(len(board)):
+        if all(row[col] == -1 for row in board):
+            return get_unmarked_sum(board)
+    return None
+
+
+def get_unmarked_sum(board):
+    return sum(n for row in board for n in row if n != -1)
+
+
+if __name__ == "__main__":
+    main()