#!/usr/bin/env python3





def read_h(typ=int):

    return list(map(typ, input().split()))





def read_v(n, typ=int):

    return [typ(eval(input())) for _ in range(n)]





def main():

    S, = read_h(str)

    flag = [False] * 26

    for c in S:

        i = ord(c) - ord('a')

        if not flag[i]:

            flag[i] = True



    ans = None

    for i, f in enumerate(flag):

        if not f:

            print((chr(i + ord('a'))))

            return



    print((str(ans)))





if __name__ == '__main__':

    main()

#!/usr/bin/env python3





def read_h(typ=int):

    return list(map(typ, input().split()))





def read_v(n, typ=int):

    return [typ(eval(input())) for _ in range(n)]





def main():

    S = eval(input())



    for i in range(26):

        c = chr(ord('a') + i)

        if c not in S:

            print(c)

            return



    print(None)





if __name__ == '__main__':

    main()

#!/usr/bin/env python3
def read_h(typ=int):
    return list(map(typ, input().split()))


def read_v(n, typ=int):
    return [typ(eval(input())) for _ in range(n)]


def main():
    (S,) = read_h(str)
    flag = [False] * 26
    for c in S:
        i = ord(c) - ord("a")
        if not flag[i]:
            flag[i] = True
    ans = None
    for i, f in enumerate(flag):
        if not f:
            print((chr(i + ord("a"))))
            return
    print((str(ans)))


if __name__ == "__main__":
    main()

#!/usr/bin/env python3
def read_h(typ=int):
    return list(map(typ, input().split()))


def read_v(n, typ=int):
    return [typ(eval(input())) for _ in range(n)]


def main():
    S = eval(input())
    for i in range(26):
        c = chr(ord("a") + i)
        if c not in S:
            print(c)
            return
    print(None)


if __name__ == "__main__":
    main()

-    (S,) = read_h(str) -    flag = [False] * 26 -    for c in S: -        i = ord(c) - ord("a") -        if not flag[i]: -            flag[i] = True -    ans = None -    for i, f in enumerate(flag): -        if not f: -            print((chr(i + ord("a")))) +    S = eval(input()) +    for i in range(26): +        c = chr(ord("a") + i) +        if c not in S: +            print(c) -    print((str(ans))) +    print(None)