You are given a string S consisting of lowercase English letters. Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in S. If every lowercase English letter occurs in S, print `None` instead.
s = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"
flag = 0
for s in alphabet:
  if s.count(s) == 0:
    print(s)
    break
  else:flag += 1

if flag ==26:print("None")
s = sorted(input())
alphabet = "abcdefghijklmnopqrstuvwxyz"
flag = 0
for t in alphabet:
  if s.count(t) == 0:
    print(t)
    break
  else:flag += 1

if flag ==26:print("None")