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)
 {
Scanner sc = new Scanner(System.in);
   int N=sc.nextInt();
   if(N%2==0) System.out.println ((N/2)-1); 
	/*int count=0;   
   HashMap<Integer> map = new HashMap<Integer>();
   for(int i=1;i<N/2;i++)
   {
	if(map.get(i)==null)
    {
		count++;
      	
    }*/
    else 
   System.out.println(N/2);
     
   }
   
 
  
}
