s = eval(input())

for c in 'abcdefghijklmnopqrstuvwxyz':

    if s.find(c) == -1:

        print(c)

        exit()

    

print('None')
s = eval(input())

for i in range(97, 123):

    if s.find(chr(i)) == -1:

        print((chr(i)))

        exit()

    

print('None')
s = eval(input())
for c in "abcdefghijklmnopqrstuvwxyz":
    if s.find(c) == -1:
        print(c)
        exit()
print("None")

s = eval(input())
for i in range(97, 123):
    if s.find(chr(i)) == -1:
        print((chr(i)))
        exit()
print("None")

-for c in "abcdefghijklmnopqrstuvwxyz": -    if s.find(c) == -1: -        print(c) +for i in range(97, 123): +    if s.find(chr(i)) == -1: +        print((chr(i)))