#include <stdio.h>
#include <iostream>
using namespace std;
int main()
{
	int n;
	cin >> n;
	int sum = 0;
	int num =1,day = 1;
	for (int i = 1 ; i <= n ; i++)
	{
		sum += num;
		day -= 1;
		if(day ==0)
		{
			num++;
			day = num;
		}
	}
	cout << sum << endl;
}