#include <iostream>
using namespace std;

int get_num(int n){
	int total = 1;
	for(int i = 0;i < n; i++){
		total += i + 1;
	}
	return total;
}

int main(){
	int n;
	while(cin >> n){
		cout << get_num(n) << endl;
	}
	return 0;
}