#include <iostream>
using namespace std;

int main() {
    int k;
    cin >> k;
    
    int total_coins = 0;
    int day = 1;
    
    while (k > 0) {
        for (int i = 1; i <= day && k > 0; i++) {
            total_coins += day;
            k--;
        }
        day++;
    }
    
    cout <<  total_coins << endl;

    return 0;
}