#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n,k=0,t=1;
	cin>>n;
	while(n!=0)
	{
		for(int i=1;i<=t;i++)
		{
			k+=t;
			n-=1;
			if(n==0)
			{
				cout<<k;
				return 0;	
			}
		}
		t++;
	}
	return 0;
}

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;
}