#include <iostream>
#include <cstdio>
using namespace std;
#define rep2(x,from,to) for(int x = (from); x < (to); ++(x))
#define rep(x,to) rep2(x,0,to)

int main() {
	int f[10001];
	f[1] = 2;
	rep2(i,2,10001) {
		f[i] = f[i-1] + i;
	}
	int n;
	while(cin >> n) {
		cout << f[n] << endl;
	}
	return 0;
}