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: # -*- coding: utf-8 -*-
  import sys
  from collections import defaultdict, deque
  
  # def input(): return sys.stdin.readline()[:-1] # warning not \n
  # def input(): return sys.stdin.buffer.readline()[:-1]
  
  
  def solve():
      s = set([x for x in input()])
      for i in range(26):
          e = chr(ord("a") + i)
          if e not in s:
              print(e)
              return
      print("None")
  
  
  t = 1
  # t = int(input())
  for case in range(1, t + 1):
      ans = solve()
  
  
  """
  
  1 4
  7
  RRRR
  1 1 0 1
  ----
  4
  
  1 4
  47
  RCRR
  1 1 0 1
  ----
  -1
  
  2 2
  10
  RR
  RR
  1 7
  4 1
  ----
  4
  
  2 2
  10
  RR
  RR
  7 4
  7 4
  ----
  -1
  
  3 3
  8
  RCR
  RRC
  RRR
  1 1 1
  1 1 1
  1 3 1
  
  
  3 3
  1
  RCR
  RRC
  RRR
  1 1 1
  1 1 1
  1 3 1
  
  
  """
  
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"}]