'''
    Python3(PyPy3) Template for Programming-Contest.

    author : sgsw

    generated : 2021/08/13   
    when : 21:45:24

'''

import sys


def input():
    return sys.stdin.readline().rstrip()


DXY = [(0, -1), (1, 0), (0, 1), (-1, 0)]  # LDRU
mod = 998244353
inf = 1 << 64


def main():
    s = list(input())
    s = list(map(lambda x: ord(x),s))
    ans = ["zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"]
    for i in range(26):
        if i + ord("a") not in s:
            ans.append(chr(ord("a") + i))
    m = min(s)
    b = s.count(m)
    c = chr(m)
    ans.append(c * (b + 1))
    ans.sort()
    print(ans[0])
    return 0


if __name__ == "__main__":
    main()