#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
using namespace std;

int main()
{
	//freopen("coin.in", "r", stdin);
	//freopen("coin.out", "w", stdout);

	int k;
	cin >> k;

	int ans = 0, cur = 1, cnt = 0;
	for (int i = 1; i <= k; ++i)
	{
		ans += cur;
		if (++cnt == cur)
		{
			++cur;
			cnt = 0;
		}
	}
	cout << ans;

	fclose(stdin);
	fclose(stdout);
	return 0;
}
/**************************************************************
	Problem: 2340
	User: admin
	Language: C++
	Result: Accepted
	Time:50 ms
	Memory:2072 kb
****************************************************************/