import sys

import os



ii = lambda: int(sys.stdin.buffer.readline().rstrip())

il = lambda: list(map(int, sys.stdin.buffer.readline().split()))

fl = lambda: list(map(float, sys.stdin.buffer.readline().split()))

iln = lambda n: [int(sys.stdin.buffer.readline().rstrip()) for _ in range(n)]



iss = lambda: sys.stdin.buffer.readline().decode().rstrip()

sl = lambda: list(map(str, sys.stdin.buffer.readline().decode().split()))

isn = lambda n: [sys.stdin.buffer.readline().decode().rstrip() for _ in range(n)]



MOD = 10 ** 9 + 7





def main():

    if os.getenv("LOCAL"):

        sys.stdin = open("input.txt", "r")



    S = list(iss())

    for n in range(ord('a'), ord('z') + 1):

        if chr(n) not in S:

            print((chr(n)))

            exit()

    else:

        print('None')





if __name__ == '__main__':

    main()

import sys

import os

import math



ii = lambda: int(sys.stdin.buffer.readline().rstrip())

il = lambda: list(map(int, sys.stdin.buffer.readline().split()))

fl = lambda: list(map(float, sys.stdin.buffer.readline().split()))

iln = lambda n: [int(sys.stdin.buffer.readline().rstrip()) for _ in range(n)]



iss = lambda: sys.stdin.buffer.readline().decode().rstrip()

sl = lambda: list(map(str, sys.stdin.buffer.readline().decode().split()))

isn = lambda n: [sys.stdin.buffer.readline().decode().rstrip() for _ in range(n)]



MOD = 10 ** 9 + 7





def main():

    if os.getenv("LOCAL"):

        sys.stdin = open("input.txt", "r")



    T = set('abcdefghijklmnopqrstuvwxyz')

    S = set(iss())

    s = sorted(T ^ S)

    print((s[0] if len(s) > 0 else 'None'))







if __name__ == '__main__':

    main()

import sys
import os

ii = lambda: int(sys.stdin.buffer.readline().rstrip())
il = lambda: list(map(int, sys.stdin.buffer.readline().split()))
fl = lambda: list(map(float, sys.stdin.buffer.readline().split()))
iln = lambda n: [int(sys.stdin.buffer.readline().rstrip()) for _ in range(n)]
iss = lambda: sys.stdin.buffer.readline().decode().rstrip()
sl = lambda: list(map(str, sys.stdin.buffer.readline().decode().split()))
isn = lambda n: [sys.stdin.buffer.readline().decode().rstrip() for _ in range(n)]
MOD = 10**9 + 7


def main():
    if os.getenv("LOCAL"):
        sys.stdin = open("input.txt", "r")
    S = list(iss())
    for n in range(ord("a"), ord("z") + 1):
        if chr(n) not in S:
            print((chr(n)))
            exit()
    else:
        print("None")


if __name__ == "__main__":
    main()

import sys
import os
import math

ii = lambda: int(sys.stdin.buffer.readline().rstrip())
il = lambda: list(map(int, sys.stdin.buffer.readline().split()))
fl = lambda: list(map(float, sys.stdin.buffer.readline().split()))
iln = lambda n: [int(sys.stdin.buffer.readline().rstrip()) for _ in range(n)]
iss = lambda: sys.stdin.buffer.readline().decode().rstrip()
sl = lambda: list(map(str, sys.stdin.buffer.readline().decode().split()))
isn = lambda n: [sys.stdin.buffer.readline().decode().rstrip() for _ in range(n)]
MOD = 10**9 + 7


def main():
    if os.getenv("LOCAL"):
        sys.stdin = open("input.txt", "r")
    T = set("abcdefghijklmnopqrstuvwxyz")
    S = set(iss())
    s = sorted(T ^ S)
    print((s[0] if len(s) > 0 else "None"))


if __name__ == "__main__":
    main()

+import math -    S = list(iss()) -    for n in range(ord("a"), ord("z") + 1): -        if chr(n) not in S: -            print((chr(n))) -            exit() -    else: -        print("None") +    T = set("abcdefghijklmnopqrstuvwxyz") +    S = set(iss()) +    s = sorted(T ^ S) +    print((s[0] if len(s) > 0 else "None"))