#include<iostream>
#include<cstdio>
#include<string>
using namespace std;
int serch(int n)
{
    if(n==1)return 1;
    else return n+serch(n-1);
}
int main()
{
    int n;
    while(cin>>n){
        n=serch(n)+1;
        cout<<n<<endl;
    }
}