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.math.*;
import java.util.*;
 
 
 
 
 
public class Main {
 
    static MyScanner in = new MyScanner();
    static   PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
    static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    static StringTokenizer st;
    static long H,W;
    static BitSet bs;
    static long mod = 1000000007;
      // All possible moves of a knight 
    static   int X[] = { 2, 1, -1, -2, -2, -1, 1, 2 }; 
    static int Y[] = { 1, 2, 2, 1, -1, -2, -2, -1 }; 
    public static void main(String args[]) throws IOException {
        int n = in.nextInt();
        if((n&1)==0)
        out.println(n/2-1);
        else
        out.println(n/2);
        out.flush();
    }
    static boolean isVowel(char c){
        if(c=='a'||c=='e'||c=='i'||c=='o'||c=='u'||c=='y'){
            return true;
        }
        return false ;
    }
    static class IP implements Comparable<IP>{
        public int first,second;
        IP(int first, int second){
            this.first = first;
            this.second = second;
        }
        public int compareTo(IP ip){
            if(first==ip.first)
                return second-ip.second;
            return first-ip.first;    
        }
 
    }
        
       static long gcd(long a, long b){
        return b!=0?gcd(b, a%b):a;
    }
 
        static boolean isEven(long a) {
            return (a&1)==0;
        }
    
    public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
   
        public MyScanner() {
           br = new BufferedReader(new InputStreamReader(System.in));
        }
   
        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
   
        int nextInt() {
            return Integer.parseInt(next());
        }
   
        long nextLong() {
            return Long.parseLong(next());
        }
   
        double nextDouble() {
            return Double.parseDouble(next());
        }
   
        String nextLine(){
            String str = "";
        try {
           str = br.readLine();
        } catch (IOException e) {
           e.printStackTrace();
        }
        return str;
        }
  
     }
}