s = sorted(eval(input()))



for i in range(26):

    if chr(i + 97) not in s:

        print((chr(i + 97)))

        exit()

print(None)

s = eval(input())



for i in range(26):

    if chr(i + 97) not in s:

        print((chr(i + 97)))

        quit()

print(None)

s = sorted(eval(input()))
for i in range(26):
    if chr(i + 97) not in s:
        print((chr(i + 97)))
        exit()
print(None)

s = eval(input())
for i in range(26):
    if chr(i + 97) not in s:
        print((chr(i + 97)))
        quit()
print(None)

-s = sorted(eval(input())) +s = eval(input()) -        exit() +        quit()