import sys

S = eval(input())

T = []

B = []

A = 'abcdefghijklmnopqrstuvwxyz'

for i in range(len(S)):

   T.append(S[i])

for i in range(len(A)):

   B.append(A[i])

for i in B:

    if not i in T:

        print(i)

        sys.exit()

print('None')
import sys

S = list(eval(input()))

D = ['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']

for d in D:

    if not d in S:

        print(d)

        sys.exit()

print('None')
import sys

S = eval(input())
T = []
B = []
A = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(S)):
    T.append(S[i])
for i in range(len(A)):
    B.append(A[i])
for i in B:
    if not i in T:
        print(i)
        sys.exit()
print("None")

import sys

S = list(eval(input()))
D = [
    "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",
]
for d in D:
    if not d in S:
        print(d)
        sys.exit()
print("None")

-S = eval(input()) -T = [] -B = [] -A = "abcdefghijklmnopqrstuvwxyz" -for i in range(len(S)): -    T.append(S[i]) -for i in range(len(A)): -    B.append(A[i]) -for i in B: -    if not i in T: -        print(i) +S = list(eval(input())) +D = [ +    "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", +] +for d in D: +    if not d in S: +        print(d)