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.*;
public class Main
{
    public static void main(String[] args)
    {
        int count = 0;
        boolean sw = false;
        int max = Integer.MIN_VALUE;
        int min = Integer.MAX_VALUE;
        double dMax = Double.MIN_VALUE;
        double dMin = Double.MAX_VALUE;
        //StringBuilder answer=new StringBuilder();
        //input
        Scanner sc = new Scanner(System.in);
        int n=sc.nextInt();
        for (int i = 1;i < (n+1)/2;++i)
        {
            int a = n-i;
            if (n!=a) ++count;
        }
        System.out.println(count);
        sc.close();
    }
}