#!/usr/bin/env python3
# from typing import *
import itertools

def solve(s: str) -> str:
    for k in itertools.count():
        if 'a' * k not in s:
            return 'a' * k


# generated by oj-template v4.8.0 (https://github.com/online-judge-tools/template-generator)
def main():
    S = input()
    a = solve(S)
    print(a)


if __name__ == '__main__':
    main()