#include<iostream>
using namespace std;
int main() {
    int k, n = 1,  sum = 0, x = 1;
    cin >> k;
    for (int i = 1; i <= k; i++) {
        if (x <= n)
            sum += n;
        else {
            sum += ++n;
            x = 1;
        }
        x++;
    }
    cout << sum;
    return 0;
}
/**************************************************************
	Problem: 2340
	User: godzzw
	Language: C
	Result: Compile Error
****************************************************************/