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.*;

class Main{
	public static void main(String[] $){
		System.err.println();
		Scanner s=new Scanner(System.in);
		int n=s.nextInt();
		System.out.println((n-1)/2);
	}
}
