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.io.PrintWriter;
import java.util.Scanner;

public class Main {

    static Scanner sc = new Scanner(System.in);
    static PrintWriter out = new PrintWriter(System.out);

    public static void main(String[] args) {
        int n = nextInt();

        int Answer = 0;

        if (n % 2 == 0) {
        	Answer = (n / 2) - 1;
        } else {
        	Answer = (n - 1) / 2;
        }

        println(Answer);
    }

    static int nextInt() {
        return Integer.parseInt(next());
    }

    static String next() {
        return sc.next();
    }

    static String nextLine(){
        return sc.nextLine();
    }

    static void println(Object o) {
        out.println(o);
        out.flush();
    }

}