output_description: Print the answer.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
attempt: a = (int)(input())
  print(a // 2)
  
problem_description: Statement
  
  How many ways are there to choose two distinct positive integers totaling N,
  disregarding the order?
samples: [{"input": "4", "output": "1\n    \n\nThere is only one way to choose two distinct integers totaling 4: to choose 1\nand 3. (Choosing 3 and 1 is not considered different from this.)\n\n* * *"}, {"input": "999999", "output": "499999"}]