The green checkmarks show which lavender squares Derek can stand on safely. The 6-dotted sides of the Dangerous Dice have holes so if those sides roll over him, he doesn't get squished! There are 15 lavender squares(shown by the checkmarks here) that he can stand on safely. The 4 striped squares and 2 checkered squares(the entrance & exit) count as safe squares, too, since the Dangerous Dice can't roll over them. Therefore, there are 21 squares in this maze that Derek can stand on safely!
All it takes to solve this puzzle is some geometric thinking.
© Derek Cumberbatch