#include<iostream>
using namespace std;

int main(){
    
    int n;
    int count = 2;
    while(){
        cin >> n;
        if(n == EOF) break;
    for(int i = 1;i<n;i++){
        
        count += i + 1; 
        if(n == 1) count = 2;
    }
    
    cout<< count <<endl;
    }

    
}