#include <bits/stdc++.h>
using namespace std;
using ill = int long long;
#define rep(i, n) for (ill i = 0; i < (n); i++)
#define all(s) s.begin(), s.end()
using inp = pair<int, int>;
using vecb = vector<bool>;
using vecb2 = vector<vecb>;
int main()
{
    int n;
    cin >> n;
    cout << (n - 1) * n / 2 << endl;
}