include

using namespace std; int main() {

int a = 0, k,b=0;
cin >> k;
for (int i = 1;; i++) {
	for (int j = 1; j <= i; j++) {
		a += i;
		b += 1;
		if (b == k) {
			cout << a << endl;
			return 0;
		}
	}
}

}