Problem:
Score : 100 points

Problem Statement

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

1 \leq N \leq 10^6

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

4

Sample Output 1

1

There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999

Sample Output 2

499999

Solution in PHP (5.6.30) (judge verdict: Runtime Error)

<?php
  $n=fgets(STDIN);
  if($n%2==0){
    echo $n/2-1;
    else{
      echo (n-1)/2;
    }
    
    
  }