diff --git a/buggy.py b/buggy.py
--- a/buggy.py
+++ b/buggy.py
@@ -24,7 +24,7 @@
         for cha in s:
             ct2 += 1
             if cha != "a":
-                ans += chr(ord(cha) - 2)
+                ans += chr(ord(cha) - 1)
                 change = True  # record operation at least once.
                 continue
             else:
@@ -33,4 +33,6 @@
                     contiguous = False
             if not contiguous:  # then break , add the remaining string to the end of the ans.
                 break
-        return ans + s[ct2:] if change else UndefinedFunc(ans[:-1] + "z")
+        return (
+            ans + s[ct2:] if change else ans[:-1] + "z"
+        )  # check if it consists entirely of the letter "a."  'aaaa' > 'aaaz'
