Easy

#include<bits/stdc++.h>
using namespace std;

int main()
{
	long long k,s=0,day=0,week=1;
	cin>>k;
	for(int i=1;i<=k;i++)
	{
		s+=week;
		day++;
		if(day==week)
		{
			week++;
			day=0;
		}
	}
	cout<<s;
	return 0;
}