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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
 
public class Main {
  public static void main(String [] args) {
    FastReader fr = new FastReader();
    int N = fr.nextInt();
    if (N % 2 != 0) {
      N = N/2;
    } else {
      N = N/2 - 1;
    }
	System.out.println(N);
  }
}
 
class FastReader {
  BufferedReader br;
  StringTokenizer st;
 
  public FastReader() {
    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;
  }
}