a = 'abcdefghijklmnopqrstuvwxyz'

s = list(eval(input()))

for i in a:

    if i not in s:

        print(i)

        exit()

print('None')

a = 'abcdefghijklnmopqrstuvwxyz'



S = eval(input())

for i in a:

    if i not in S:

        print(i)

        exit(0)

print('None')
a = "abcdefghijklmnopqrstuvwxyz"
s = list(eval(input()))
for i in a:
    if i not in s:
        print(i)
        exit()
print("None")

a = "abcdefghijklnmopqrstuvwxyz"
S = eval(input())
for i in a:
    if i not in S:
        print(i)
        exit(0)
print("None")

-a = "abcdefghijklmnopqrstuvwxyz" -s = list(eval(input())) +a = "abcdefghijklnmopqrstuvwxyz" +S = eval(input()) -    if i not in s: +    if i not in S: -        exit() +        exit(0)