diff --git a/buggy.py b/buggy.py
--- a/buggy.py
+++ b/buggy.py
@@ -23,6 +23,6 @@
             else:
                 if a[i] != c[i]:
                     cnt += 1
-                if d[i] != c[i]:
+                if b[i] != c[i]:
                     cnt += 1
         return cnt
