#include <iostream>
using namespace std;

int main()
{
	int n;
	while(cin >> n){
		int area=0;
		if(n==0)
			area = 0;
		else if(n==1)
			area = 2;
		else if(n==2)
			area = 4;
		else if(n==3)
			area = 7;
		else{
			area = 7;
			for(int i=3;i<n;i++){
				area += i+1;
			}
		}
		cout << area << endl;
	}
	return 0;
}