import sys

s = sorted(eval(input()))

a = ord('a')

for j in range(26):

    b = a

    for i in s:

        if a == ord(i):

            a += 1

            break

    if a == b:

        print((chr(a)))

        sys.exit()

print('None')

S=sorted(set(eval(input())))

ans=None

for i in range(97,97+26):

  if chr(i) not in S:

    ans=chr(i)

    break

print(ans)

import sys

s = sorted(eval(input()))
a = ord("a")
for j in range(26):
    b = a
    for i in s:
        if a == ord(i):
            a += 1
            break
    if a == b:
        print((chr(a)))
        sys.exit()
print("None")

S = sorted(set(eval(input())))
ans = None
for i in range(97, 97 + 26):
    if chr(i) not in S:
        ans = chr(i)
        break
print(ans)

-import sys - -s = sorted(eval(input())) -a = ord("a") -for j in range(26): -    b = a -    for i in s: -        if a == ord(i): -            a += 1 -            break -    if a == b: -        print((chr(a))) -        sys.exit() -print("None") +S = sorted(set(eval(input()))) +ans = None +for i in range(97, 97 + 26): +    if chr(i) not in S: +        ans = chr(i) +        break +print(ans)