ALPHABET = "abcdefghijklmnopqrstuvwxyz"



s = list(eval(input()))

s.sort()

for i in ALPHABET:

    if i not in s:

        print(i)

        break

else:

    print("None")

ALPHABET = "abcdefghijklmnopqrstuvwxyz"



s = list(eval(input()))

for i in ALPHABET:

    if i not in s:

        print(i)

        break

else:

    print("None")

ALPHABET = "abcdefghijklmnopqrstuvwxyz"
s = list(eval(input()))
s.sort()
for i in ALPHABET:
    if i not in s:
        print(i)
        break
else:
    print("None")

ALPHABET = "abcdefghijklmnopqrstuvwxyz"
s = list(eval(input()))
for i in ALPHABET:
    if i not in s:
        print(i)
        break
else:
    print("None")

-s.sort()