A few years ago Sajjad left his school and register to another one due to security reasons. Now he wishes to find Amir, one of his schoolmates and good friends.

There are n schools numerated from 1 to n. One can travel between each pair of them, to do so, he needs to buy a ticket. The ticker between schools i and j costs <image> and can be used multiple times. Help Sajjad to find the minimum cost he needs to pay for tickets to visit all schools. He can start and finish in any school.

Input

The first line contains a single integer n (1 ≤ n ≤ 105) — the number of schools.

Output

Print single integer: the minimum cost of tickets needed to visit all schools.

Examples

Input

2


Output

0


Input

10


Output

4

Note

In the first example we can buy a ticket between the schools that costs <image>.
import java.util.*;
import java.io.*;

public class PA {

    /*

        Tyger! Tyger! burning bright
        In the forests of the night,
        What immortal hand or eye
        Could frame thy fearful symmetry?

        In what distant deeps or skies
        Burnt the fire of thine eyes?
        On what wings dare he aspire?
        What the hand, dare sieze the fire?

        And what shoulder, & what art,
        Could twist the sinews of thy heart?
        And when thy heart began to beat,
        What dread hand? & what dread feet?

        What the hammer? what the chain?
        In what furnace was thy brain?
        What the anvil? what dread grasp
        Dare its deadly terrors clasp?

        When the stars threw down their spears,
        And water’d heaven with their tears,
        Did he smile his work to see?
        Did he who made the Lamb make thee?

        Tyger! Tyger! burning bright
        In the forests of the night,
        What immortal hand or eye
        Dare frame thy fearful symmetry?

    */
    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter out = new PrintWriter(System.out);

        int n = Integer.parseInt(br.readLine());

        if(n % 2 !=- 0)
        {
            out.println(n / 2);
        }
        else
        {
            out.println(n/2 - 1);
        }

        out.flush();
        out.close();

    }

}