#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repp(i, n) rep(i, 0, n)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define int long long
using namespace std;
typedef pair<int, int> P;

signed main(){
    int n;
    cin >> n;
    // while(n % 2 == 0){
    //     n /= 2;
    // }
    // cout << (n % 2 == 1 ? n : 1) << endl;
    cout << n * (n - 1) / 2 << endl;
}