Subtract Points when click button.

hi there, i am fallowing the unity tutorial on how to roll a ball ( im very new on this) i was able to do it everything, but in my practice i create a button that make the ball roll and score points. was just wondering how substract points when i first click the button. something like, "you loose one point to roll a ball but win 2 points when collect" what should i change in the tutorial code to be able to do it. here is the link for the tutorial https://unity3d.com/learn/tutorials/projects/roll-ball-tutorial/collecting-pick-objects?playlist=17141