#include <iostream>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <numeric>
#include <utility>

#define rep(i,n)	for(int i = 0;i < n;i++)
#define rep2(i,n)	for(int i = 1;i <= n;i++)
#define each(i,x)	for(auto & i : x)

using namespace std;

int main()
{
	while(1)
	{
		int n;
		cin >> n;
		if(cin.eof())
		{
			break;
		}
		int k = 1;
		rep2(j,n)
		{
			k += j;
		}
		cout << k << endl;
	}
	return 0;
}