#include<bits/stdc++.h>
using namespace std;
int k;
int ans;
int now;
int pre;
signed main()
{
	cin>>k;
	pre=1;
	for(int i=1;i<=k;i++)
	{
		now++;
		ans+=pre;
		if(now==pre)
		{
			pre++;
			now=0;
		}
	}
	cout<<ans;
	return 0;
}