#include <iostream>
using namespace std;

int main() {
    int days;
    cin >> days;

    int total_coins = 0;
    int current_days = 0;
    int current_coins = 1;

    while (current_days < days) {
        for (int i = 0; i < current_coins; i++) {
            if (current_days < days) {
                total_coins += current_coins;
                current_days++;
            } else {
                break;
            }
        }
        current_coins++;
    }

    cout << total_coins << endl;

    return 0;
}    
/**************************************************************
	Problem: 1520
	User: panyuchen
	Language: C++
	Result: Accepted
	Time:46 ms
	Memory:2072 kb
****************************************************************/