#include<iostream>
using namespace std;
int ans[10001];
int main()
{
	ans[1]=2;
	for(int i = 2; i < 10001; ++i)
	{
		ans[i]=ans[i-1]+i;
	}

	int n;
	while (cin>>n) 
	{
		cout<<ans[n]<<endl;
	}
  return 0;
}