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) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int M = 0;
		if(N % 2 == 0) {
			M = N / 2;
		}else {
			M = N / 2 + 1;
		}

		int counter = 0;
		for(int i = 1; i < M; i++) {
			if( (N - i) != i) {
				counter++;
			}else {

			}
		}
		System.out.println(counter);
	}

}
