Doing a bit of guessing here, but if you mean the two variables you set at the top of your screenshot: they won’t work for maths because you’re filling them with the Item object. If you want to get the state of the Item then you need to use the get state of Item block.
Okay, even if I get the state of the item it doesn’t work. I think because the state of an item is text and not a number. Hence you can’t do maths with text. I couldn’t see how to convert it to a number. That is the root of the question. e.g.