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++14 (GCC 5.4.1) (judge verdict: Accepted)

/*input
*/
#include <bits/stdc++.h>
using namespace std;
 
#define all(a) a.begin(), a.end()
#define ff first
#define ss second
#define pb push_back
 
using ll = long long;
using pii = pair<int, int>;
 
template<typename T> using V = vector<T>;

int main() {
	ios::sync_with_stdio(false);cin.tie(nullptr);
	ll n; cin >> n;
	--n;
	cout << (n*(n+1))/2;
}