#include <cstdio>

using namespace std;

int main()
{
    int n;
    while( scanf( "%d", &n ) != EOF )
    {
        if( n == 1 )
        {
            printf( "%d\n", 2 );
        }
        else
        {
            printf( "%d\n", ( ( n + 1 ) * n + 2 ) / 2 );
        }
    }
    return 0;
}
