import string

al = string.ascii_lowercase



S = set(eval(input()))



for i in al:

  if i not in S:

    print(i)

    exit()



print("None")
S = set(eval(input()))

Flag = [True]*26



for i in S:

  Flag[ord(i)-97] = False



for i in range(26):

  if Flag[i]:

    print((chr(97+i)))

    exit()



print("None")
import string

al = string.ascii_lowercase
S = set(eval(input()))
for i in al:
    if i not in S:
        print(i)
        exit()
print("None")

S = set(eval(input()))
Flag = [True] * 26
for i in S:
    Flag[ord(i) - 97] = False
for i in range(26):
    if Flag[i]:
        print((chr(97 + i)))
        exit()
print("None")

-import string - -al = string.ascii_lowercase -for i in al: -    if i not in S: -        print(i) +Flag = [True] * 26 +for i in S: +    Flag[ord(i) - 97] = False +for i in range(26): +    if Flag[i]: +        print((chr(97 + i)))