S = eval(input())

for c in 'abcdefghijklmnopqrstuvwxyz':

  if S.count(c) == 0:

    print(c)

    break

else:

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

alf = 'abcdefghijklmnopqrstuvwxyz'

for c in alf:

  if c not in s:

    print(c)

    break

else:

  print('None')
S = eval(input())
for c in "abcdefghijklmnopqrstuvwxyz":
    if S.count(c) == 0:
        print(c)
        break
else:
    print("None")

s = eval(input())
alf = "abcdefghijklmnopqrstuvwxyz"
for c in alf:
    if c not in s:
        print(c)
        break
else:
    print("None")

-S = eval(input()) -for c in "abcdefghijklmnopqrstuvwxyz": -    if S.count(c) == 0: +s = eval(input()) +alf = "abcdefghijklmnopqrstuvwxyz" +for c in alf: +    if c not in s: