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 C (GCC 5.4.1) (judge verdict: Accepted)

#include <limits.h>
#include <math.h>
#include <stdio.h>
int main(void) {
  int n;
  int cnt=0;
  scanf("%d",&n);
  // for (int i = 1; i < n; i++)
  // {

    
  // }
  if(n%2){
printf("%d\n",(n/2));
  }else{
    printf("%d\n",(n/2)-1);
  }
  

  return 0;
}