#include<iostream>
using namespace std;

int Calc( int x )
{
    if( x == 1 )
        return 2;
    
    return x + Calc( x - 1 );
}

int main()
{
    int n;

    while( cin >> n )
    {
        cout << Calc( n ) << endl;
    }
    return 0;
}
