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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;
using System.Numerics;

namespace nik20102
{
    class Program
    {
        static void Main(string[] args)
        {
            //Console.SetIn(new StreamReader("input.txt"));
            solve_nik20102A();
        }

        public static void solve_nik20102B()
        {

        }

        public static void solve_nik20102A()
        {
            int n = Convert.ToInt32(Console.ReadLine());

            Console.WriteLine(n % 2 == 0 ? n / 2 - 1 : n / 2);
        }
    }
}