Does anyone have a simple way to reverse the value of a slider variable?

The answer is probably staring me in the face but here goes ...

I have a slider which starts at 0 and ends at 100. Initial position is 0, step increment is 1. When I move the slider, its position variable (var1) correctly displays 0-100 as it moves.

But I also want a another variable (var2) which goes from 100 - 0 as the slider is moved from 0-100. Ideally, I should be able to add a Trigger which adjusts the value of var2 = 100-var1 but Storyline 3 doesn't offer this basic operation.

Any suggestions gratefully received!

Thanks, Tim