import string



S = str(eval(input()))

ans = "None"



for s in string.ascii_lowercase:

    if s not in S:

        ans = s

        break



print(ans)

S = eval(input())

ans = "None"

alphabet = [chr(ord("a") + i) for i in range(26)]



for s in alphabet:

    if s not in S:

        ans = s

        break



print(ans)

import string

S = str(eval(input()))
ans = "None"
for s in string.ascii_lowercase:
    if s not in S:
        ans = s
        break
print(ans)

S = eval(input())
ans = "None"
alphabet = [chr(ord("a") + i) for i in range(26)]
for s in alphabet:
    if s not in S:
        ans = s
        break
print(ans)

-import string - -S = str(eval(input())) +S = eval(input()) -for s in string.ascii_lowercase: +alphabet = [chr(ord("a") + i) for i in range(26)] +for s in alphabet: