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 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        long N = GetInputLong();

        N -= 1;
        double sum = 0;

        sum = (N / 2d * (N + 1));

        Console.Write(sum);

        Console.ReadKey();
    }


    public static int GetInputInt()
    {
        return int.Parse(Console.ReadLine());
    }
    public static int[] GetInputsInt()
    {
        return Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
    }
    public static List<int> GetInputsIntList()
    {
        return Console.ReadLine().Split(' ').Select(int.Parse).ToList();
    }
    public static List<float> GetInputsFloatList()
    {
        return Console.ReadLine().Split(' ').Select(float.Parse).ToList();
    }

    public static long GetInputLong()
    {
        return long.Parse(Console.ReadLine());
    }
    public static long[] GetInputsLong()
    {
        return Console.ReadLine().Split(' ').Select(long.Parse).ToArray();
    }

}