a = list(eval(input()))

aa = list(set(a))

alpha = [chr(i) for i in range(97,97+26)]





if len(aa) == 26:

    print('None')

else:

    for i in range(26):

        if alpha[i] not in a:

            print((alpha[i]))

            break
s = eval(input())

s = set(s)



for a in 'abcdefghijklmnopqrstuvwxyz':

    if a not in s:

        print(a)

        break

else:

    print('None')
a = list(eval(input()))
aa = list(set(a))
alpha = [chr(i) for i in range(97, 97 + 26)]
if len(aa) == 26:
    print("None")
else:
    for i in range(26):
        if alpha[i] not in a:
            print((alpha[i]))
            break

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

-a = list(eval(input())) -aa = list(set(a)) -alpha = [chr(i) for i in range(97, 97 + 26)] -if len(aa) == 26: +s = eval(input()) +s = set(s) +for a in "abcdefghijklmnopqrstuvwxyz": +    if a not in s: +        print(a) +        break +else: -else: -    for i in range(26): -        if alpha[i] not in a: -            print((alpha[i])) -            break