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.

Examples

Input

4


Output

1


Input

999999


Output

499999
import java.util.*;
public class Main {

    public static void main(String[] args) {
        //data input
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        long ct = 0;

        for (int i = 1; i < n; i++) {
            int m = n - i;
            if (m <= i) break;
            if (m + i == n) ct++;
        }

        System.out.println(ct);
    }
}