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.Scanner;

public class Main {

    public static void main(String[] args) {
	Scanner scan = new Scanner(System.in);
	int a = scan.nextInt();
	if (a % 2 == 0) {
	    a = a - 1;
	}
	System.out.println((a-1)/2);
	scan.close();
    }

}