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: Wrong Answer)

#include <stdio.h>
int main(){
	int n, m, x, y, i;
	if(scanf("%d", &n)== 1);
	m = 1;
	i = (unsigned int)(n/2);
  	for(x=1; x<=i; ++x){
  		y = n - x;
		if(y % 2 == 0){
          if(n - y != y){
			++m;
          }
		}
  	}

	printf("%d\n", m);
    
	return 0;
}