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)

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <set>
#include <algorithm>
#include <array>
#include <complex>
#include <string>
#include <utility>
#include <map>
#include <queue>
#include <list>
#include <functional>
#include <numeric>
#include <stack>
#include <tuple>
#include <fstream>
using namespace std;
int dx[4] = { -1,0,1,0 };
int dy[4] = { 0,1,0,-1 };
const int INF = 100000000;
const int BINF = 100000000000000001;
const long long LINF = 1000000000000000000;
const int MOD = (int)1e9 + 7;
const double EPS = 1e-6;
const double pi = 3.14159265359;
using pii = std::pair<int, int>;
using ll = long long;
using pLL = std::pair<ll, ll>;
#define SORT(v) std::sort(v.begin(), v.end())
#define RSORT(v) std::sort(v.rbegin(), v.rend())


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N;
    cin >> N;
    ll ans = 0;
    for (int i = 1; i < N; ++i)ans += i;
    cout << ans << endl;
    return 0;
}