diff --git a/day04/sol.py b/day04/sol.py
index 0d1e768..29f284e 100644
--- a/day04/sol.py
+++ b/day04/sol.py
@@ -1,4 +1,5 @@
 import sys
+from collections import defaultdict
 
 def matches2score(n):
     if n <= 0:
@@ -7,15 +8,30 @@ def matches2score(n):
 
 input = sys.stdin
 part1 = 0
+card = 0
+copies = defaultdict(int)
 for line in input:
+    card += 1
     _, _, data  = line.partition(": ")
     winners, numbers = data.split("|")
     winners = map(int, winners.split())
     numbers = map(int, numbers.split())
 
     matches = len(set(winners) & set(numbers))
+
     s = matches2score(matches)
     print(s)
     part1 += s
 
+    copies[card] += 1 # original copy
+    for i in range(matches):
+        copies[card+i+1] += copies[card]
+
+print("-")
+
 print(part1)
+
+assert not [i for i,c in copies.items() if c > 0 and i > card]
+
+part2 = sum(copies.values())
+print(part2)