output_description: Print the lexicographically smallest lowercase English letter that does not
  occur in S. If every lowercase English letter occurs in S, print `None`
  instead.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      S
attempt: from __future__ import print_function
  
  import sys
  
  import re
  import array
  import copy
  import functools
  import operator
  
  import math
  import string
  import fractions
  from fractions import Fraction
  from fractions import gcd
  
  # fractions.gcd() を用いること
  # math.log2()はatcoderでは対応していない．留意せよ．
  
  import collections
  import itertools
  import bisect
  
  import random
  import time
  
  import heapq
  from heapq import heappush
  from heapq import heappop
  from heapq import heappushpop
  from heapq import heapify
  from heapq import heapreplace
  from queue import PriorityQueue as pq
  from queue import Queue
  
  from itertools import accumulate
  
  from collections import deque
  from collections import Counter
  
  from operator import mul
  from functools import reduce
  
  input = sys.stdin.readline
  
  
  def eprint(*args, **kwargs):
      print(*args, file=sys.stderr, **kwargs)
      return
  
  
  def lcm(n, m):
      return int(n * m / gcd(n, m))
  
  
  def coprimize(p, q):
      common = fractions.gcd(p, q)
      return (p // common, q // common)
  
  
  def combinations_count(n, r):
      r = min(r, n - r)
      numer = reduce(mul, range(n, n - r, -1), 1)
      denom = reduce(mul, range(1, r + 1), 1)
      return numer // denom
  
  
  def find_gcd(list_l):
      x = reduce(gcd, list_l)
      return x
  
  
  def main():
      S = set(list("abcdefghijklmnopqrstuvwxyz")) - set(list(input().strip()))
      if len(list(S)) != 0:
          print(sorted(list(S))[0])
      else:
          print("None")
  
  
  if __name__ == "__main__":
      main()
  
problem_description: Statement
  
  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.
samples: [{"input": "atcoderregularcontest", "output": "b\n    \n\nThe string `atcoderregularcontest` contains `a`, but does not contain `b`.\n\n* * *"}, {"input": "abcdefghijklmnopqrstuvwxyz", "output": "None\n    \n\nThis string contains every lowercase English letter.\n\n* * *"}, {"input": "fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg", "output": "d"}]