N=3. I have a different setup, though. I have my original glasses to drive (-3.25, but they are now too strong, I need to downgrade them). Then I have glasses for other uses (-1, IIRC. They allow me to see at the limit of bluriness at 2-3 meters). And for computer work, I wear no glasses at all. Worth noting, though, that before I started, my prescription hadn't changed in 25 years.