#include <iostream>
using namespace std;

int main(){
	int ans[10001];
	ans[0] = 1;
	for(int i = 1; i < 10001; i++){
		ans[i] = ans[i-1] + i;
	}
	int n;
	while(cin >> n){
		cout << ans[n] << endl;
	}
	return 0;
}