CF708A Letters Cyclic Shift

You are given a non-empty string $ s $ consisting of lowercase English letters. You have to pick exactly one non-empty substring of $ s $ and shift all its letters 'z'  'y'  'x'  'b'  'a'  'z'. In other words, each character is replaced with the previous character of English alphabet and 'a' is replaced with 'z'. What is the lexicographically minimum string that can be obtained from $ s $ by performing this shift exactly once?

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String $ s $ is lexicographically smaller than some other string $ t $ of the same length if there exists some $ 1