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;

class Main{
  public static void main(String args[]){
    int N;
    Scanner sc = new Scanner(System.in);
    
    N = sc.nextInt();
    if (N % 2 == 0){
      System.out.println(N / 2 - 1);
    }else{
      System.out.println((N-1)/2);
    }
  }
}