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 i,N,ans;
    scanf("%d",&N);
    if(N==1 || N==2){
        ans = 0;
    }
    else if(N%2==0){
        ans = N/2 - 1;
    } 
    else if(N%2==1){
        ans = N/2;
    }
    printf("%d\n",ans);
    return 0;
}