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[] args) {
    Scanner sc = new Scanner(System.in);
    int n = Integer.parseInt(sc.next());
    int ans = 0;
    int nh = 0;
    
    if(n%2==0){
      nh = n/2;
    } else {
      nh = (n+1)/2;
    }
    
    for(int i=1; i<nh; i++){
      if(n-i!=i){
        ans++;
      }
    }

    System.out.println(ans);
    
  }
}
