#include <iostream> using namespace std; int main() { int n; cin >> n; if (n <= 0) { cout << 0 << endl; return 0; } if (n == 1 || n == 2) { cout << 1 << endl; return 0; } long long current = 1; for (int i = 2; i <= n; i++) { current += (i - 1); } cout << current << endl; return 0; }