#include <bits/stdc++.h> using namespace std; #define ll long long //#define DBG int main() { #ifdef DBG freopen("flower.in", "r", stdin); freopen("flower.out", "w", stdout); #endif ll llN = 0; cin >> llN; cout << llN*(llN-1)/2 << endl; #ifdef DBG fclose(stdin); fclose(stdout); #endif return 0; }