#include <iostream>
#include <string>
#include <math.h>
#include <algorithm>


using namespace std;
int main(){

    int x;
    while(cin>>x){
        int count=0;
        for(int i=1; i<=x; i++){
            if(i==1){
                count=2;
            }else{
                count+=i;
            }
        }
        cout<<count<<endl;
    }

}