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.io.*;
import java.util.*;

public class Main {
    static FastScanner in;
    static PrintWriter out;
    static final int MOD = 1000000007;

    public static void main(String[] args) throws IOException {
        //        Scanner in = new Scanner(new File("input.txt"));
        //        Scanner in = new Scanner(System.in);
        //        System.setOut(new PrintStream(new BufferedOutputStream(new FileOutputStream("output.txt")), true));
        out = new PrintWriter(System.out);
        in = new FastScanner(System.in);
        //        in = new FastScanner("input.txt");
        int n = in.nextInt();
        int i = 1, j = n;
        while (i < j) {
            if (i == j - 1)
                break;
            i++;
            j--;
        }
        out.println(i - 1);
        out.close();
    }
}

class Pair {
    public int first; //first member of pair
    public int second; //second member of pair

    public Pair(int first, int second) {
        this.first = first;
        this.second = second;
    }

    public Pair() {

    }
}

class FastScanner {

    BufferedReader br;
    StringTokenizer tokenizer;

    FastScanner(String fileName) throws FileNotFoundException {
        this(new FileInputStream(new File(fileName)));
    }

    FastScanner(InputStream is) {
        br = new BufferedReader(new InputStreamReader(is));
    }

    String nextLine() throws IOException {
        tokenizer = null;
        return br.readLine();
    }

    String next() throws IOException {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            String line = br.readLine();
            if (line == null) {
                return null;
            }
            tokenizer = new StringTokenizer(line);
        }
        return tokenizer.nextToken();
    }

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

    long nextLong() throws IOException {
        return Long.parseLong(next());
    }

    double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    char nextChar() throws IOException {
        return next().charAt(0);
    }
}