#include<cstdio>
#include<cmath>
#include<algorithm>
#include<iostream>
#include<string>
#include <vector>
#include <map>
#include<cstdlib>

using namespace std;

void Solution()
{
    int n;
    
    while(cin>>n)
    {
       //algorithm
       cout<<(n * (n + 1) / 2 + 1)<<endl; // 1+1 2+2 4+3 7+4 ....... (1+2+4)7 + (n = )4
    }
}

int main()
{
    Solution();
    return 0;
}