import string

s = set(string.ascii_lowercase) - set(eval(input()))

print((min(s) if s else 'None'))
S = eval(input())

for c in 'abcdefghijklmnopqrstuvwxyz':

    if c not in S:

        print(c)

        exit()

print('None')
import string

s = set(string.ascii_lowercase) - set(eval(input()))
print((min(s) if s else "None"))

S = eval(input())
for c in "abcdefghijklmnopqrstuvwxyz":
    if c not in S:
        print(c)
        exit()
print("None")

-import string - -s = set(string.ascii_lowercase) - set(eval(input())) -print((min(s) if s else "None")) +S = eval(input()) +for c in "abcdefghijklmnopqrstuvwxyz": +    if c not in S: +        print(c) +        exit() +print("None")