Checkbox to add/subtract the total amount

Checkbox to add/subtract the total amount

I have a form which has basic computation like adding and subtracting the amount of money to be inputted by user. Now, everything works fine, except the "Penalty" field, I want the user to click the checkbox (checked) if they want to add the amount in Penalty field in "Total Payment" field and uncheck  to remove the amount of penalty field in total payment.

and also is it possible, if the penalty field doesnt have amount, and the user clicked the checkbox it will return a notification saying please input the penalty amount then it will go back to uncheck field?

Please see my works, http://jsfiddle.net/php988/7g4vxy2L/

help would be appreciated,
Thanks