Variables to define learner progress

Hello team, I have what might be an easy solution. My team would like to show on a slide master the following:

You are on slide X out of Y slides. Where X is (in a linear fashion), which slide the learner is viewing and Y is the total number of slides.

I believe Project.TotalSlides is the variable for Y. What variable would represent X?

Is it Project.SlideReference or Project.SlideNumber?

Thanks!