Have slider subtract from one variable to add to itself.

Is it possible to have a slider to subtract from a variable to add to itself and check if there's anything left to take? I'm trying to assign skill points to skills using a slider because it seems quicker then the player hitting a button 50 times to set points. So I need it to check if there's anything left in my pointsLeft variable when it tries to go up and then subtract from that variable. Should also ask if I can group it with other sliders so that each subtracts from that variable and if there's nothing left to subtract from one another.