Problem:
Score : 100 points

Problem Statement

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.

Sample Input 1

4

Sample Output 1

1

There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999

Sample Output 2

499999

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main {
	
    private static Integer count = 0;
      
	public static void main(String[] args) {

		// argumentの取得
		Scanner sc = new Scanner(System.in);

		int x = sc.nextInt();
      
        if (x == 1) {
          System.out.println(0);
          return;
        }
      
        if ((x % 2) == 0) {
         //System.out.println(x + " は偶数");
         System.out.println((x-2)/2);
        } else {
         //System.out.println(x + " は奇数");
         System.out.println((x-1)/2);
        }
	} 
}