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 N = sc.nextInt();
		sc.close();

		int cnt = 0;
		for(int i = 1;i <= N/2;i++){
			cnt++;
		}
		if(N==1 || N==2){
			System.out.println(0);
			return;
		}
		System.out.println(N%2==0 ? --cnt : cnt);
	}
}