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: def counting_sort(array, max):
      bucket = [0] * (max + 1)
  
      for i in range(len(array)):
          if array[i] == "a":
              bucket[0] += 1
          elif array[i] == "b":
              bucket[1] += 1
          elif array[i] == "c":
              bucket[2] += 1
          elif array[i] == "d":
              bucket[3] += 1
          elif array[i] == "e":
              bucket[4] += 1
          elif array[i] == "f":
              bucket[5] += 1
          elif array[i] == "g":
              bucket[6] += 1
          elif array[i] == "h":
              bucket[7] += 1
          elif array[i] == "i":
              bucket[8] += 1
          elif array[i] == "j":
              bucket[9] += 1
          elif array[i] == "k":
              bucket[10] += 1
          elif array[i] == "l":
              bucket[11] += 1
          elif array[i] == "m":
              bucket[12] += 1
          elif array[i] == "n":
              bucket[13] += 1
          elif array[i] == "o":
              bucket[14] += 1
          elif array[i] == "p":
              bucket[15] += 1
          elif array[i] == "q":
              bucket[16] += 1
          elif array[i] == "r":
              bucket[17] += 1
          elif array[i] == "s":
              bucket[18] += 1
          elif array[i] == "t":
              bucket[19] += 1
          elif array[i] == "u":
              bucket[20] += 1
          elif array[i] == "v":
              bucket[21] += 1
          elif array[i] == "w":
              bucket[22] += 1
          elif array[i] == "x":
              bucket[23] += 1
          elif array[i] == "y":
              bucket[24] += 1
          elif array[i] == "z":
              bucket[25] += 1
      return bucket
  
  
  def main():
      S = input()
      bucket_list = counting_sort(S, 25)
      for i in range(26):
          if bucket_list[i] == 0:
              if i == 0:
                  print("a")
                  return
              if i == 1:
                  print("b")
                  return
              if i == 2:
                  print("c")
                  return
              if i == 3:
                  print("d")
                  return
              if i == 4:
                  print("e")
                  return
              if i == 5:
                  print("f")
                  return
              if i == 6:
                  print("g")
                  return
              if i == 7:
                  print("h")
                  return
              if i == 8:
                  print("i")
                  return
              if i == 9:
                  print("j")
                  return
              if i == 10:
                  print("k")
                  return
              if i == 11:
                  print("l")
                  return
              if i == 12:
                  print("m")
                  return
              if i == 13:
                  print("n")
                  return
              if i == 14:
                  print("o")
                  return
              if i == 15:
                  print("p")
                  return
              if i == 16:
                  print("q")
                  return
              if i == 17:
                  print("r")
                  return
              if i == 18:
                  print("s")
                  return
              if i == 19:
                  print("t")
                  return
              if i == 20:
                  print("u")
                  return
              if i == 21:
                  print("v")
                  return
              if i == 22:
                  print("w")
                  return
              if i == 23:
                  print("x")
                  return
              if i == 24:
                  print("y")
                  return
              if i == 25:
                  print("z")
                  return
      print("None")
  
  
  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"}]