Count total sum with quantity

Hello!

I'm very new to Bizagi, sorry for stupid question.

I have two entities. First - parameter with type of material and cost, second - connected with first and contains quantity. Second entity is also connected with main entity. As you can see on 2nd pic, form contains type, where user can choose the type, than corresponding price will apeard beacause I made filter, than user should enter quantity. And than I want to count total cost, but I don't know how. For this purpouse I made extra atribute in second entity.

Also I have table with the same idea in this form and I'd like to count total cost of the form.