import string

s = list(eval(input()))

alpha = list(string.ascii_lowercase)

n = len(alpha)

ans = "None"

for i in range(n):

  if alpha[i] not in s:

    ans = alpha[i]

    break



print(ans)
import sys

s=eval(input())

for i in range(ord("a"),ord("z")+1):

  if chr(i) not in s:

    print((chr(i)))

    sys.exit()

print("None")
import string

s = list(eval(input()))
alpha = list(string.ascii_lowercase)
n = len(alpha)
ans = "None"
for i in range(n):
    if alpha[i] not in s:
        ans = alpha[i]
        break
print(ans)

import sys

s = eval(input())
for i in range(ord("a"), ord("z") + 1):
    if chr(i) not in s:
        print((chr(i)))
        sys.exit()
print("None")

-import string +import sys -s = list(eval(input())) -alpha = list(string.ascii_lowercase) -n = len(alpha) -ans = "None" -for i in range(n): -    if alpha[i] not in s: -        ans = alpha[i] -        break -print(ans) +s = eval(input()) +for i in range(ord("a"), ord("z") + 1): +    if chr(i) not in s: +        print((chr(i))) +        sys.exit() +print("None")