include <bits/stdc++.h>

using namespace std;

int main() {

int k;
cin >> k;
int a = 0;
int n = 1;
int days = 1;
while (days <= k) {
	for (int i = 1; i <= n; i++) {
		a = a + n;
		days++;
		if (days > k) {
			break;
		}

	}
	n++;
}
cout << a;
return 0;

}