Horizontal Sum Product

Good Afternoon All

I have attached an excel file to illistrate

I am wanting to sum the product of several pairs of columns to a cell right at the end. The problem is when i need to add more pairs of columns (Stock 4, Stock 5, Stock 6) etc, it will become extremely painful to add. I feel that a SumProduct is the way to go, however i cannot see how to get it to work in this situation.

I would appreciate any assistance to make the resulting formula easier to program when adding additional stocks will be easier.

Thanks in advance.