#include <iostream>
#include <string>
using namespace std;

int main(){
	int n;
	int hoge;
	//int sum = 0;
	//while(cin >>n&& n){
	/*if(n%2==0)hoge = n/2;
	else hoge = n/2+1;*/
		int sum = 2;
		int piyo = 2;
	for(int i=1; i<n; ++i){
		sum += piyo;
		piyo++;
	}
	cout << sum << endl;
	//}
	return 0;
}