diff --git a/2021/day12.py b/2021/day12.py
new file mode 100644
index 0000000..fb043eb
--- /dev/null
+++ b/2021/day12.py
@@ -0,0 +1,45 @@
+import copy
+import fileinput
+from collections import Counter, defaultdict
+
+
+graph = defaultdict(set)
+smalls = set()
+
+for line in fileinput.input():
+    a, b = line.strip().split('-')
+
+    graph[a].add(b)
+    graph[b].add(a)
+
+
+def dfs(start, extra_time=False):
+    num_paths = [0]
+    def _dfs(node, seen=None, doubled=False):
+        if seen is None:
+            seen = Counter({'start': 1 if extra_time else 0})
+
+        if node == 'end':
+            num_paths[0] += 1
+            return
+
+        # If small cave, check if part 1/2 and skip accordingly.
+        if node == node.lower():
+            if (doubled or not extra_time) and seen[node] >= 1:
+                return
+            elif not doubled and seen[node] >= 2:
+                return
+
+        seen[node] += 1
+        if node == node.lower() and node not in ('start', 'end') and seen[node] >= 2:
+            doubled = True
+
+        for n in graph[node]:
+            _dfs(n, copy.deepcopy(seen), doubled=doubled)
+
+    _dfs(start)
+    return num_paths[0]
+
+
+print "Part 1:", dfs('start')
+print "Part 2:", dfs('start', extra_time=True)