s_in = eval(input())

s = set(s_in)

lst = [ord(i) - ord('a') for i in s]

lst.sort()

for i in range(26):

    if not i in lst:

        print((chr(i + ord('a'))))

        break

    elif i == 25:

        print('None')

s = eval(input())

for i in range(ord('a'), ord('z') + 1):

  if not chr(i) in s:

    print((chr(i)))

    break

  if i == ord('z'):

    print('None')

s_in = eval(input())
s = set(s_in)
lst = [ord(i) - ord("a") for i in s]
lst.sort()
for i in range(26):
    if not i in lst:
        print((chr(i + ord("a"))))
        break
    elif i == 25:
        print("None")

s = eval(input())
for i in range(ord("a"), ord("z") + 1):
    if not chr(i) in s:
        print((chr(i)))
        break
    if i == ord("z"):
        print("None")

-s_in = eval(input()) -s = set(s_in) -lst = [ord(i) - ord("a") for i in s] -lst.sort() -for i in range(26): -    if not i in lst: -        print((chr(i + ord("a")))) +s = eval(input()) +for i in range(ord("a"), ord("z") + 1): +    if not chr(i) in s: +        print((chr(i))) -    elif i == 25: +    if i == ord("z"):