# -*- coding: utf-8 -*-

s = list(str(eval(input())))



alph = ['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']

if len(set(s)) >= 26:

    print("None")

    exit()

for i in alph:

    for j in s:

        if not(i in s):

            print(i)

            exit()

# -*- coding: utf-8 -*-

s = list(str(eval(input())))



alph = ['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']

"""

if len(set(s)) >= 26:

    print("None")

    exit()

for i in alph:

    for j in s:

        if not(i in s):

            print(i)

            exit()

"""



check = [False] * 26

for i in s:

    for j in range(26):

        if i == alph[j]:

            check[j] = True

tmp = 0

for i in range(26):

    if check[i] == False:

        print((alph[i]))

        exit()

    if check[i]:

        tmp += 1

if tmp == 26:

    print("None")

# -*- coding: utf-8 -*-
s = list(str(eval(input())))
alph = [
    "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",
]
if len(set(s)) >= 26:
    print("None")
    exit()
for i in alph:
    for j in s:
        if not (i in s):
            print(i)
            exit()

# -*- coding: utf-8 -*-
s = list(str(eval(input())))
alph = [
    "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",
]
"""
if len(set(s)) >= 26:
    print("None")
    exit()
for i in alph:
    for j in s:
        if not(i in s):
            print(i)
            exit()
"""
check = [False] * 26
for i in s:
    for j in range(26):
        if i == alph[j]:
            check[j] = True
tmp = 0
for i in range(26):
    if check[i] == False:
        print((alph[i]))
        exit()
    if check[i]:
        tmp += 1
if tmp == 26:
    print("None")

+""" -        if not (i in s): +        if not(i in s): +""" +check = [False] * 26 +for i in s: +    for j in range(26): +        if i == alph[j]: +            check[j] = True +tmp = 0 +for i in range(26): +    if check[i] == False: +        print((alph[i])) +        exit() +    if check[i]: +        tmp += 1 +if tmp == 26: +    print("None")