S = list(eval(input()))

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

for char in alpha:

    if char not in S:

        print(char)

        break

else:

    print(None)

S = set(eval(input()))

for a in "abcdefghijklmnopqrstuvwxyz":

    if a not in S:

        print(a)

        break

else:

    print("None")

S = list(eval(input()))
alpha = [chr(ord("a") + i) for i in range(26)]
for char in alpha:
    if char not in S:
        print(char)
        break
else:
    print(None)

S = set(eval(input()))
for a in "abcdefghijklmnopqrstuvwxyz":
    if a not in S:
        print(a)
        break
else:
    print("None")

-S = list(eval(input())) -alpha = [chr(ord("a") + i) for i in range(26)] -for char in alpha: -    if char not in S: -        print(char) +S = set(eval(input())) +for a in "abcdefghijklmnopqrstuvwxyz": +    if a not in S: +        print(a) -    print(None) +    print("None")