diff --git a/lib/helpers.rb b/lib/helpers.rb
index 7029df9..3e21a8d 100644
--- a/lib/helpers.rb
+++ b/lib/helpers.rb
@@ -19,7 +19,7 @@ helpers do
   end
 
   def serialize(num)
-    return num
+    return num.to_s.rjust(4, '0')
   end
 
 end