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.
# encoding:utf-8
import copy
import random
import bisect 
import math


S = str(input())

print(S)
all = list("abcdefghijklmnopqrstu")
for i in range(len(S)):
    if S[i] in all:
        all.remove(S[i])

if len(all) > 0:
    print(all[0])
else:
    print("None")

#!/usr/bin/env python3
# encoding:utf-8
import copy
import random
import bisect 
import fractions 
import math
import sys
import collections
from decimal import Decimal 

mod = 10**9+7
sys.setrecursionlimit(mod) 

d = collections.deque()
def LI(): return list(map(int, sys.stdin.readline().split()))

S = str(input())
al = list("abcdefghijklmnopqrstuvwxyz")

for i in al:
    if i in S:
        pass
    else:
        print(i)
        break
else:
    print("None")