s = eval(input())

alfa = [chr(i) for i in range(97, 97+26)]

for i in range(len(alfa)):

    for j in range(len(s)):

        if alfa[i] == s[j]:

            alfa[i] = "-"

for i in range(len(alfa)):

    if alfa[i] != "-":

        ans = alfa[i]

        break

    else:

        ans = "None"

print(ans)
import sys



S = eval(input())

alfa = [chr(i) for i in range(97, 97+26)]

l = [0] * 26



for i in range(len(S)):

    index = alfa.index(S[i])

    l[index] += 1



for i in range(len(l)):

    if l[i] == 0:

        print((alfa[i]))

        sys.exit()

print("None")

s = eval(input())
alfa = [chr(i) for i in range(97, 97 + 26)]
for i in range(len(alfa)):
    for j in range(len(s)):
        if alfa[i] == s[j]:
            alfa[i] = "-"
for i in range(len(alfa)):
    if alfa[i] != "-":
        ans = alfa[i]
        break
    else:
        ans = "None"
print(ans)

import sys

S = eval(input())
alfa = [chr(i) for i in range(97, 97 + 26)]
l = [0] * 26
for i in range(len(S)):
    index = alfa.index(S[i])
    l[index] += 1
for i in range(len(l)):
    if l[i] == 0:
        print((alfa[i]))
        sys.exit()
print("None")

-s = eval(input()) +import sys + +S = eval(input()) -for i in range(len(alfa)): -    for j in range(len(s)): -        if alfa[i] == s[j]: -            alfa[i] = "-" -for i in range(len(alfa)): -    if alfa[i] != "-": -        ans = alfa[i] -        break -    else: -        ans = "None" -print(ans) +l = [0] * 26 +for i in range(len(S)): +    index = alfa.index(S[i]) +    l[index] += 1 +for i in range(len(l)): +    if l[i] == 0: +        print((alfa[i])) +        sys.exit() +print("None")