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<stdio.h>
int main()
{

    int a;
    scanf("%d", &a);
  	if(a <= 2){
      printf("0");
      return 0;
    }
  	int ans = 0;
  	if(a %2 == 0){
		ans = a/2 -1;
    }else{
      a = a+1;
      ans = a/2 -1;
    }
    
  printf("%d",ans);
    return 0;
}