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;

// https://atcoder.jp/contests/abc140/tasks/abc140_b
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();
		sc.close();

		int ans = 0;

		for(int i=1;i<=N/2;i++) {
			if(i!=(N-i)) {
				ans = ans + 1;
			}
		}


		System.out.println(ans);

	}
}
