Increment Variable only on first time click of a button

Hi,

I'm trying to come up with an action wherein if the user clicks a button, a variable (say 'v_userscore' is incremented by 1). However, I only want this variable to increment by 1 when the user clicks the first time on a slide. So if on the given slide, the user clicks anywhere else (apart from the button) first, and then clicks the button, I do not want the variable to increment by 1 because this was not the first thing that user clicked on, on that slide. Can anyone help me figure this one out?

Thanks,

Sean