What is the number supposed to be? My first thought would be that you are doing the math wrong. If that is the case then you probably need to change the order of subtraction.
If it is possible to get both negative and positive values from the calculation (and the math is actually correct) and you need to show only the positive value then you would use take the absolute value of the value with abs(). I doubt that this is the case though so I suggest just fixing your math.