text = eval(input())

alphabet = 'abcdefghijklmnopqrstuvwxyz'



for s in text:

    alphabet = alphabet.replace(s, '')



if alphabet is '':

    print('None')

else:

    print((alphabet[0]))
alpha = [chr(ord('a') + i) for i in range(26)]



S = set(eval(input()))



if len(set(alpha)) == len(S):

    print('None')

else:

    for s in alpha:

        if s not in S:

            print(s)

            exit()
text = eval(input())
alphabet = "abcdefghijklmnopqrstuvwxyz"
for s in text:
    alphabet = alphabet.replace(s, "")
if alphabet is "":
    print("None")
else:
    print((alphabet[0]))

alpha = [chr(ord("a") + i) for i in range(26)]
S = set(eval(input()))
if len(set(alpha)) == len(S):
    print("None")
else:
    for s in alpha:
        if s not in S:
            print(s)
            exit()

-text = eval(input()) -alphabet = "abcdefghijklmnopqrstuvwxyz" -for s in text: -    alphabet = alphabet.replace(s, "") -if alphabet is "": +alpha = [chr(ord("a") + i) for i in range(26)] +S = set(eval(input())) +if len(set(alpha)) == len(S): -    print((alphabet[0])) +    for s in alpha: +        if s not in S: +            print(s) +            exit()