Stop negative numbers in numeric variable

I've set up a numeric variable to countdown from 9 - 0 to represent the number of drag items to use in a drag and drop.  

Each drag item has a trigger to say subtract 1 from variable when user drops item.

There is a trigger to return variable to 9 if user gets try again layer.

The issue I am having is if the user changes their mind before they hit the submit button and drags to another drop item i then get negative numbers if already at 0 or subtracts too many if not all cards have moved. Is there a way to stop double counting?