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;
class Program
{
	static void Main(string[] args)
	{
		// 整数の入力
		int N = int.Parse(Console.ReadLine());
		
      	// スペース区切りの整数の入力
		//string[] input = Console.ReadLine().Split(' ');
      	//int[] H = new int[N];
      	//for (int i = 0;i<N;i++){
          //H[i] = int.Parse(input[i]); 
        //}
		//int A = int.Parse(input[0]);
		//int B = int.Parse(input[1]);
		// 文字列の入力
		//string s = Console.ReadLine();
		//出力
      	
		Console.WriteLine(N * (N-1) / 2);
	}
}