Easiest way to deal with it is to add a process file in the specific script, where you correct the input value to a positive number. I would say the best place to that would be in the label AP_IMPORT which is called just after the details were read from the file and table values has been loaded.  

Hi Robert,

 if the suggested answer helped, please do mark it as verified  (by clicking the "more" button on the response) for the benefit of others in this forum :). Thank you!