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;
class ProgramA
    {
        static void Main(string[] args)
        {
            
            string input = Console.ReadLine();
            int n = int.Parse(input);
          
            if(n % 2 == 0)
                Console.WriteLine((n -2)/2);
            else
            {
                Console.WriteLine((n -1)/2);
            }
        }
    }