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: import sys
  
  
  class alphabet:  # Trueなら大文字
      def __init__(self, capitalize):
          self.num = dict()  # アルファベットを数字に変換
          self.abc = [
              "a",
              "b",
              "c",
              "d",
              "e",
              "f",
              "g",
              "h",
              "i",
              "j",
              "k",
              "l",
              "m",
              "n",
              "o",
              "p",
              "q",
              "r",
              "s",
              "t",
              "u",
              "v",
              "w",
              "x",
              "y",
              "z",
          ]
          if capitalize:
              for i in range(26):
                  self.abc[i] = self.abc[i].upper()
          for i, a in enumerate(self.abc):
              self.num[a] = True
  
  
  def solve():
      S = input()
      AB = alphabet(False)
      for i in range(len(S)):
          AB.num[S[i]] = False
      for a in AB.abc:
          if AB.num[a]:
              print(a)
              break
      else:
          print("None")
  
      return 0
  
  
  if __name__ == "__main__":
      solve()
  
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"}]