output_description: Print the number of different PIN codes Takahashi can set.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      S
attempt: _, j = open(0)
  a = set(), set(), set(), {""}
  for s in j:
      for b, c in zip(a, a[1:]):
          b |= {t + s for t in c}
  print(len(a[0]))
  
problem_description: Statement
  
  AtCoder Inc. has decided to lock the door of its office with a 3-digit PIN
  code.
  
  The company has an N-digit lucky number, S. Takahashi, the president, will
  erase N-3 digits from S and concatenate the remaining 3 digits without
  changing the order to set the PIN code.
  
  How many different PIN codes can he set this way?
  
  Both the lucky number and the PIN code may begin with a 0.
samples: [{"input": "4\n    0224", "output": "3\n    \n\nTakahashi has the following options:\n\n  * Erase the first digit of S and set `224`.\n  * Erase the second digit of S and set `024`.\n  * Erase the third digit of S and set `024`.\n  * Erase the fourth digit of S and set `022`.\n\nThus, he can set three different PIN codes: `022`, `024`, and `224`.\n\n* * *"}, {"input": "6\n    123123", "output": "17\n    \n\n* * *"}, {"input": "19\n    3141592653589793238", "output": "329"}]