#include <bits/stdc++.h>
#define rep(i,a,n) for(int i=a;i<n;i++)
#define repr(i,a,n) for(int i=a;i>=n;i--)
#define INF 999999999
#define INF_M 2147483647
#define pb(a) push_back(a)
using namespace std;
typedef pair<int, int> pii;
typedef long long int ll;

int main() {
	int n;
	while(cin >> n) {
		cout << n * (n + 1) / 2 + 1 << endl;
	}
}