s1 = sorted(eval(input()))

s2 = "abcdefghijklmnopqrstuvwxyz"



for s in s2:

  if s in s1:

    pass

  else:

    print(s)

    break

else:

  print("None")
S = eval(input())



for a in "abcdefghijklmnopqrstuvwxyz":

  if a not in S:

    print(a)

    exit()



print("None")
s1 = sorted(eval(input()))
s2 = "abcdefghijklmnopqrstuvwxyz"
for s in s2:
    if s in s1:
        pass
    else:
        print(s)
        break
else:
    print("None")

S = eval(input())
for a in "abcdefghijklmnopqrstuvwxyz":
    if a not in S:
        print(a)
        exit()
print("None")

-s1 = sorted(eval(input())) -s2 = "abcdefghijklmnopqrstuvwxyz" -for s in s2: -    if s in s1: -        pass -    else: -        print(s) -        break -else: -    print("None") +S = eval(input()) +for a in "abcdefghijklmnopqrstuvwxyz": +    if a not in S: +        print(a) +        exit() +print("None")