#include <iostream>
using namespace std;
int k;
int main()
{
    cin >> k;
    long long anssss = 0;
    int a = 1,day = 1;//a为每天的工资
    for (int i = 1;i <= k;i++)
    {
        anssss += a;
        day--;
        if (day == 0)
        {
            a++;
            day = a;
        }
    }
    cout << anssss;
    return 0;
}//不是AI写的