#include <iostream>
using namespace std;

int main(){
    int n, c;
    while(cin >> n){
        //f(n+1) = f(n) + n + 1
        //f(n) = 2 + 2 + 3 + 4 ... n = 1 + n(n+1)/2
        c = n * (n+1) / 2 + 1;
        cout << c << endl;
    }
    return 0;
}