output_description: Print the first three characters of the label of the N-th round of AtCoder
  Beginner Contest.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
attempt: N = int(input())
  # 使い切ったブロックの数
  block = (N - 1) // 999
  
  a = chr(ord("C") + block)
  N2 = N - block * 999
  
  if N2 < 10:
      c = "00"
  elif N2 < 100:
      c = "0"
  else:
      c = ""
  
  
  b = "AB" + a + c + str(N2)
  
  print("{}".format(b))
  
problem_description: Statement
  
  Decades have passed since the beginning of AtCoder Beginner Contest.
  
  The contests are labeled as `ABC001`, `ABC002`, ... from the first round, but
  after the 999-th round `ABC999`, a problem occurred: how the future rounds
  should be labeled?
  
  In the end, the labels for the rounds from the 1000-th to the 1998-th are
  decided: `ABD001`, `ABD002`, ..., `ABD999`.
  
  You are given an integer N between 1 and 1998 (inclusive). Print the first
  three characters of the label of the N-th round of AtCoder Beginner Contest.
samples: [{"input": "999", "output": "ABC\n    \n\nThe 999-th round of AtCoder Beginner Contest is labeled as `ABC999`.\n\n* * *"}, {"input": "1000", "output": "ABD\n    \n\nThe 1000-th round of AtCoder Beginner Contest is labeled as `ABD001`.\n\n* * *"}, {"input": "1481", "output": "ABD\n    \n\nThe 1481-th round of AtCoder Beginner Contest is labeled as `ABD482`."}]