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 sc = new Scanner(System.in);
		int i = sc.nextInt();
		if (1 <= i && i <= 1000000) {
			if (i % 2 == 0) {
				i = i / 2 - 1;
			} else {
				i = (i - 1) / 2;
			}
		}
        System.out.println(i);
		sc.close();
	}
}
