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: hoge = input()
  flag = 0
  if flag == 0 and hoge.count("a") == 0:
      print("a")
      flag = 1
  
  if flag == 0 and hoge.count("b") == 0:
      print("b")
      flag = 1
  
  if flag == 0 and hoge.count("c") == 0:
      print("c")
      flag = 1
  
  if flag == 0 and hoge.count("d") == 0:
      print("d")
      flag = 1
  
  if flag == 0 and hoge.count("e") == 0:
      print("e")
      flag = 1
  
  if flag == 0 and hoge.count("f") == 0:
      print("f")
      flag = 1
  
  if flag == 0 and hoge.count("g") == 0:
      print("g")
      flag = 1
  
  if flag == 0 and hoge.count("h") == 0:
      print("h")
      flag = 1
  
  if flag == 0 and hoge.count("i") == 0:
      print("i")
      flag = 1
  
  if flag == 0 and hoge.count("j") == 0:
      print("j")
      flag = 1
  
  if flag == 0 and hoge.count("k") == 0:
      print("k")
      flag = 1
  
  if flag == 0 and hoge.count("l") == 0:
      print("l")
      flag = 1
  
  if flag == 0 and hoge.count("m") == 0:
      print("m")
      flag = 1
  
  if flag == 0 and hoge.count("n") == 0:
      print("n")
      flag = 1
  
  if flag == 0 and hoge.count("o") == 0:
      print("o")
      flag = 1
  
  if flag == 0 and hoge.count("p") == 0:
      print("p")
      flag = 1
  
  if flag == 0 and hoge.count("q") == 0:
      print("q")
      flag = 1
  
  if flag == 0 and hoge.count("r") == 0:
      print("r")
      flag = 1
  
  if flag == 0 and hoge.count("s") == 0:
      print("s")
      flag = 1
  
  if flag == 0 and hoge.count("t") == 0:
      print("t")
      flag = 1
  
  if flag == 0 and hoge.count("u") == 0:
      print("u")
      flag = 1
  
  if flag == 0 and hoge.count("v") == 0:
      print("v")
      flag = 1
  
  if flag == 0 and hoge.count("w") == 0:
      print("w")
      flag = 1
  
  if flag == 0 and hoge.count("x") == 0:
      print("x")
      flag = 1
  
  if flag == 0 and hoge.count("y") == 0:
      print("y")
      flag = 1
  
  if flag == 0 and hoge.count("z") == 0:
      print("z")
      flag = 1
  
  if flag == 0:
      print("None")
  
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"}]