s = eval(input())

s = sorted(s)

ascii_lowercase = 'abcdefghijklmnopqrstuvwxyz'

ans = 'None'

for chr_alpha in ascii_lowercase:

  if chr_alpha not in s:

    ans = chr_alpha

    break

print(ans)
def main():

    s = eval(input())

    s = sorted(s)

    ascii_lowercase = 'abcdefghijklmnopqrstuvwxyz'

    ans = 'None'

    for chr_alpha in ascii_lowercase:

        if chr_alpha not in s:

            ans = chr_alpha

            break

    print(ans)





if __name__ == "__main__":

    main()

s = eval(input())
s = sorted(s)
ascii_lowercase = "abcdefghijklmnopqrstuvwxyz"
ans = "None"
for chr_alpha in ascii_lowercase:
    if chr_alpha not in s:
        ans = chr_alpha
        break
print(ans)

def main():
    s = eval(input())
    s = sorted(s)
    ascii_lowercase = "abcdefghijklmnopqrstuvwxyz"
    ans = "None"
    for chr_alpha in ascii_lowercase:
        if chr_alpha not in s:
            ans = chr_alpha
            break
    print(ans)


if __name__ == "__main__":
    main()

-s = eval(input()) -s = sorted(s) -ascii_lowercase = "abcdefghijklmnopqrstuvwxyz" -ans = "None" -for chr_alpha in ascii_lowercase: -    if chr_alpha not in s: -        ans = chr_alpha -        break -print(ans) +def main(): +    s = eval(input()) +    s = sorted(s) +    ascii_lowercase = "abcdefghijklmnopqrstuvwxyz" +    ans = "None" +    for chr_alpha in ascii_lowercase: +        if chr_alpha not in s: +            ans = chr_alpha +            break +    print(ans) + + +if __name__ == "__main__": +    main()