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: #!usr/bin/env python3
  from collections import defaultdict
  from collections import deque
  from heapq import heappush, heappop
  import sys
  import math
  import bisect
  import random
  import itertools
  
  sys.setrecursionlimit(10**5)
  stdin = sys.stdin
  
  
  def LI():
      return list(map(int, stdin.readline().split()))
  
  
  def LF():
      return list(map(float, stdin.readline().split()))
  
  
  def LI_():
      return list(map(lambda x: int(x) - 1, stdin.readline().split()))
  
  
  def II():
      return int(stdin.readline())
  
  
  def IF():
      return float(stdin.readline())
  
  
  def LS():
      return list(map(list, stdin.readline().split()))
  
  
  def S():
      return list(stdin.readline().rstrip())
  
  
  def IR(n):
      return [II() for _ in range(n)]
  
  
  def LIR(n):
      return [LI() for _ in range(n)]
  
  
  def FR(n):
      return [IF() for _ in range(n)]
  
  
  def LFR(n):
      return [LI() for _ in range(n)]
  
  
  def LIR_(n):
      return [LI_() for _ in range(n)]
  
  
  def SR(n):
      return [S() for _ in range(n)]
  
  
  def LSR(n):
      return [LS() for _ in range(n)]
  
  
  mod = 1000000007
  alp = [chr(ord("a") + i) for i in range(26)]
  
  
  # A
  def A():
      x, a, b = LI()
      if abs(x - a) > abs(x - b):
          print("B")
      else:
          print("A")
      return
  
  
  # B
  def B():
      s = S()
      s = set(s)
      for i in s:
          alp.remove(i)
      if len(alp) == 0:
          print("None")
      else:
          print(alp[0])
      return
  
  
  # C
  def C():
      return
  
  
  # D
  def D():
      return
  
  
  # E
  def E():
      return
  
  
  # F
  def F():
      return
  
  
  # G
  def G():
      return
  
  
  # H
  def H():
      return
  
  
  # Solve
  if __name__ == "__main__":
      B()
  
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"}]