Comment by colonCapitalDee

It's never actually explained how n=2 is possible. Here's how it works:

P1 and P2 start executing. P1 sets temp=1, then P2 runs through the loop for i=1..9. Then P1 sets n=temp=1. P2 sets temp=2, then P1 resumes executing for i=2..10, and completes. P2 sets n=temp=2, then completes.

The intuition is P1 checkpoints, P2 runs through the loop up until the last iteration, P1 resets n to the checkpoint + 1, P2 checkpoints, P1 runs through the loop, then P2 resets n to the checkpoint + 1.