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.
ALPHABET = "abcdefghijklmnopqrstuvwxyz"

s = input()
for i in range(26):
    if s.count(ALPHABET[i]):
        print(ALPHABET[i])
        break
else:
    print("None")

from collections import Counter

S = list(input())
ac = Counter(S)

alp = 'abcdefghijklmnopqrstuvwxyz'

for a in alp:
    if not(a in ac):
        print(a)
        break
else:
    print('None')
