# ABC072-B

S = eval(input())

ss = set()

for i in range(len(S)):

    ss.add(S[i])



alpha_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 

                       'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

ans = ''

for k in range(len(alpha_list)):

    if alpha_list[k] not in ss:

        ans = alpha_list[k]

        break

if len(ss) == 26:

    print('None')

else:

    print(ans)
S = eval(input())

S = set(list(S))

alp = [chr(ord('a') + i) for i in range(26)]

for al in alp:

    if al in S:

        pass

    else:

        print(al)

        exit()

print("None")
# ABC072-B
S = eval(input())
ss = set()
for i in range(len(S)):
    ss.add(S[i])
alpha_list = [
    "a",
    "b",
    "c",
    "d",
    "e",
    "f",
    "g",
    "h",
    "i",
    "j",
    "k",
    "l",
    "m",
    "n",
    "o",
    "p",
    "q",
    "r",
    "s",
    "t",
    "u",
    "v",
    "w",
    "x",
    "y",
    "z",
]
ans = ""
for k in range(len(alpha_list)):
    if alpha_list[k] not in ss:
        ans = alpha_list[k]
        break
if len(ss) == 26:
    print("None")
else:
    print(ans)

S = eval(input())
S = set(list(S))
alp = [chr(ord("a") + i) for i in range(26)]
for al in alp:
    if al in S:
        pass
    else:
        print(al)
        exit()
print("None")

-# ABC072-B -ss = set() -for i in range(len(S)): -    ss.add(S[i]) -alpha_list = [ -    "a", -    "b", -    "c", -    "d", -    "e", -    "f", -    "g", -    "h", -    "i", -    "j", -    "k", -    "l", -    "m", -    "n", -    "o", -    "p", -    "q", -    "r", -    "s", -    "t", -    "u", -    "v", -    "w", -    "x", -    "y", -    "z", -] -ans = "" -for k in range(len(alpha_list)): -    if alpha_list[k] not in ss: -        ans = alpha_list[k] -        break -if len(ss) == 26: -    print("None") -else: -    print(ans) +S = set(list(S)) +alp = [chr(ord("a") + i) for i in range(26)] +for al in alp: +    if al in S: +        pass +    else: +        print(al) +        exit() +print("None")