Problem:
Score : 400 points

Problem Statement

For an integer N, we will choose a permutation \{P_1, P_2, ..., P_N\} of \{1, 2, ..., N\}.

Then, for each i=1,2,...,N, let M_i be the remainder when i is divided by P_i.

Find the maximum possible value of M_1 + M_2 + \cdots + M_N.

Constraints

N is an integer satisfying 1 \leq N \leq 10^9.

Input

Input is given from Standard Input in the following format:

N

Output

Print the maximum possible value of M_1 + M_2 + \cdots + M_N.

Sample Input 1

2

Sample Output 1

1

When the permutation \{P_1, P_2\} = \{2, 1\} is chosen, M_1 + M_2 = 1 + 0 = 1.

Sample Input 2

13

Sample Output 2

78

Sample Input 3

1

Sample Output 3

0

Solution in C# (Mono-csc 3.5.0) (judge verdict: Wrong Answer)


using System;
using System.Collections.Generic;
using System.Linq;
class Test
{
    public static void Main()
    {

        var n = int.Parse(Console.ReadLine());
        int sum = 0;
        if (n == 1)
        {
            sum = 0;

        }
        else if (n == 2)
        {
            sum = 1;
        }
        else
            sum = (n - 1) * (n) / 2;


        Console.Write(sum);
    }
}