diff --git a/2017/day12.py b/2017/day12.py
new file mode 100644
index 0000000..dd62e4f
--- /dev/null
+++ b/2017/day12.py
@@ -0,0 +1,28 @@
+from collections import defaultdict
+
+with open("input/day12.txt") as f:
+    input = f.read()
+
+neighbors = defaultdict(set)
+
+for line in input.splitlines():
+    a, bs = line.split(" <-> ")
+    for b in bs.split(", "):
+        neighbors[a].add(b)
+        neighbors[b].add(a)
+
+
+def connected(start, neighbors) -> int:
+    queue = [start]
+    seen = set()
+    while queue:
+        node = queue.pop(0)
+        if node in seen:
+            continue
+
+        seen.add(node)
+        queue.extend(neighbors[node])
+    return len(seen)
+
+
+print(connected("0", neighbors))