s = eval(input())

alp = set("abcdefghijklmnopqrstuvwxyz")

print((sorted(list(alp - set(s)))[0] if len(alp - set(s)) != 0 else 'None'))
S = set(eval(input()))

s = list('abcdefghijklmnopqrstuvwxyz')

for i in s:

    if i not in S:

        print(i)

        exit()

print('None')
s = eval(input())
alp = set("abcdefghijklmnopqrstuvwxyz")
print((sorted(list(alp - set(s)))[0] if len(alp - set(s)) != 0 else "None"))

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

-s = eval(input()) -alp = set("abcdefghijklmnopqrstuvwxyz") -print((sorted(list(alp - set(s)))[0] if len(alp - set(s)) != 0 else "None")) +S = set(eval(input())) +s = list("abcdefghijklmnopqrstuvwxyz") +for i in s: +    if i not in S: +        print(i) +        exit() +print("None")