using System;

public class Program

{

    public static int Count( int n)

    {

        return ((n * (n+1))/2) - n;

    }

    public static int OldCount2(int n)

     {

        int pair = 0;

​

        if((n< 2) || (n> 10000))

        {

            return 0;

        }

        //pair = 1;

        for (int i = 1; i<= n ; i++)

        {

            pair += i;

        }

        return (pair - n);

     }

    public static int OldCount1(int n)

     {

         int pair = 0;

​

         if((n< 2) || (n> 10000))

         {

            return 0;

         }

​

         for (int i = 0; i< n ; i++)

         {

            for(int j = i+1; j<n; j++)

            {

            pair++;

            }

         }

         return pair;

     }

    public static void Main()

    {

        //int n = 4;

        int n = 10000;

        Console.WriteLine(Count(n));

        //Console.WriteLine(OldCount(n));

    }

}