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.*;
import java.io.*;

public class Main {
	private static void solve(){
	//Implement solution here.
		int n = ni();		
		System.out.println(n % 2 == 0 ? (n/2) - 1 : ((n + 1)/2) - 1);
	}

    //Switch input source (stdin/file)
	private static BufferedReader br  = new BufferedReader(new InputStreamReader(System.in));
	public static void main(String[] args){
        String debugDataPath = System.getenv("DD_PATH");        
        if(debugDataPath != null){
            try{
            	br = new BufferedReader(new InputStreamReader(new FileInputStream(new File(debugDataPath))));
            }catch(Exception e){
                throw new RuntimeException(e);
            }
        }
        solve();
	}

    //Input read utility
	private static StringTokenizer tokenizer = null;
	public static String next() {
		while (tokenizer == null || !tokenizer.hasMoreTokens()) {
			try {
				tokenizer = new StringTokenizer(br.readLine());
			} catch (Exception e) {
				throw new RuntimeException(e);
			}
		}
		return tokenizer.nextToken();
	}
    //Get next single int
	private static int ni() {
		return Integer.parseInt(next());
	}
    //Get next single long
	private static long nl() {
		return Long.parseLong(next());
	}
    //Get next single double
	private static double nd() {
		return Double.parseDouble(next());
	}
    //Get next int array from one line
	private static int[] na(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = ni();
		return a;
	}
	//Get next char array from one line
	private static char[] ns() {
		return next().toCharArray();
	}
    //Get next long array from one line
	private static long[] nal(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)
			a[i] = nl();
		return a;
	}
}