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>.
//package March16;

import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.StringTokenizer;
 
public class Main{
	public static void main(String[] args) throws java.lang.Exception{
		InputStream inputstream = System.in;
		OutputStream outputstream = System.out;
		InputReader in = new InputReader(inputstream);
		PrintWriter out = new PrintWriter(outputstream);
		Call one = new Call();
		one.solve(in,out);
		out.close();
	}
	static class Call {
		/*
		 	RISK
		 
		 	And then the day came,
			when the risk
			to remain tight
			in a bud
			was more painful
			than the risk
			it took
			to blossom.
		 */
		public void solve(InputReader in,PrintWriter out) {	
			int n = in.nextInt();
			out.print((n-1)/2);
		}
	}
	static class InputReader {
		public BufferedReader reader;
		public StringTokenizer tokenizer;
		public InputReader(InputStream stream) {
			reader = new BufferedReader(new InputStreamReader (stream),32768);
			tokenizer = null;
		}
		public String next() {
				while(tokenizer == null || !tokenizer.hasMoreElements()) {
					try {
						tokenizer = new StringTokenizer(reader.readLine());
					}
					catch(IOException e) {
						throw new RuntimeException(e);
					}
				}
			return tokenizer.nextToken();
		}
		public int nextInt() {
			return Integer.parseInt(next());
		}
		public long nextLong() {
			return Long.parseLong(next());
		}
	}
}
