#include <iostream>
#include <algorithm>
#include <vector>
#include <cmath>
#include <cstdio>
using namespace std;

// long long int answer[10002];
long long int ans(int in, long long int answer[]){
	if (in == 1){
		answer[in] = 2;
		return answer[1];
	}
	answer[in] = ans(in - 1, answer) + in;
	return answer[in];
}

int main(){
	int in,result;

	// ans(10000, answer);
	while (cin >> in, !cin.eof()){
		result = 1;
		for (int i = 0; i < in; i++){
			// +1, +2, +3,..
			result += (i+1);
		}
		cout << result << endl;
	}

	return 0;
}