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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

class Program
{
    static void Main(string[] args)
    {
        string str = Console.ReadLine();
        //string[] str = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得
        //string str = "<>>><<><<<<<>>><";
        decimal A = Int64.Parse(str);
        //int B = int.Parse(str[1]);
        //string[] strHeight = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得
        //string[] results = new string[N];
        //long[] result = new long[str.Length+1];
        //result[0] = 0;
        //long min = 0;
        //long minpoint = 0;

        if( A % 2 == 0)
        {
            Console.WriteLine(A / 2);
        }
        else
        {
            Console.WriteLine(Math.Floor(A / 2));
        }
    }
}