#include <iostream>

using namespace std;

int main()
{
    int k; //输入发放金币的天数
    int data = 1; //动态发放金币天数
    int sum=0; //发放金币总数
    cin >> k;
    for (int i = 1; data <= k; i++)
    {
        for (int j = 0; j < i; j++)
        {
            if (data > k)
            {
                break;
            }
            sum += i;
            data++;
        }
    }
    cout << sum << endl;
}