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 debug(String str) {
        System.out.println(str);
    }

    public static void main(String[] args) {

        long N;

        try (Scanner sc = new Scanner(System.in)) {
            N = sc.nextLong();
        }
        long result = N /2;
        if(N%2 == 0){
            result--;
        }
        System.out.println(result);

    }

}
