#include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <map>
#include <set>
#include <iostream>
#include <climits>
#include <cfloat>


using namespace std;


int main()
{
	int area[10001];
	area[0] = 1;
	for (int i = 1; i <= 10000; ++i)
		area[i] = area[i-1] + i;

	int n;
	while (scanf("%d\n", &n) != EOF)
		printf("%d\n", area[n]);

	return 0;
}