It'd be better to just have 2 bricks both 1/3 of the way(s) through than scanning for every single brick:
You'd need to do the 2 bricks consecutively, you can't skip either. A "lap" will count only if you touched the second brick.
If you touched 1/3 and you'd go back, nothing would happen.
If you touched 1/3 then 2/3, you can continue and fulfill the lap.
If you touched 1/3, then 2/3 and decided to go back, because you'd be forced to touch 1/3 you aren't going to get the lap through reverse, as touching 1/3 would "cancel" the 2/3.