diff --git a/day05/src/main.rs b/day05/src/main.rs
index 73f47d9..c8d9c63 100644
--- a/day05/src/main.rs
+++ b/day05/src/main.rs
@@ -38,3 +38,5 @@ fn is_nice_2(s: &&str) -> bool {
 
     subrule_1 && subrule_2
 }
+
+// https://adventofcode.com/2015/day/5