I'm not sure if it's the right place to ask this question but I have a mathematical problem with net carb calculation I have no idea how to solve!

Here's the problem:

I am in late stage of OWL and am little by little adding tortilla flat bread to my meal. As an illustration, I have tortilla bread (17.8 gram carb, 3.5 gram fiber), cheddar cheese, and some herbs as lunch. I make Margarita pizza this way. A bit later, as dessert, I have a chocolate bar (3.3gr carbohydrate, 10gr fiber, 9 gr Erythritol) and a cup of tea. The question is, how much net carb is there in this meal?

I know that net carb can be calculated by the simple formula carbohydrate- fiber- sugar alcohols. Yet, putting it this way it would be somehow too good to be true that I just have consumed -1.4 net carb!:confused:

Is there anybody who can help me with this problem?

I'd be grateful!

Cheers