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
{
  void Solve()
  {
    var N = ToInts()[0];
    Console.WriteLine(N*(N-1)/2); ;
  }
  static int[] ToInts()
  {
    var ss = Console.ReadLine().Split();
    var ret = new /**/int[ss.Length];
    for (int i=0;i<ss.Length ;i++)
    {
      ret[i] = /**/int.Parse(ss[i]);
    }
    return ret;
  }

  static void Main(string[] args)
  {
    new Program().Solve();
  }
}