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;

namespace nikkei2019_2_qual_a
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = decimal.Parse(Console.ReadLine());
            var r = Math.Floor(N / 2);
            Console.WriteLine(N % 2 == 0 ? r - 1 : r);
        }
    }
}