23. The courses at the magic school are divided into 9 levels, and students start learning from level 1. After completing level 9, they can become magicians. During the learning process, skipping levels is allowed, but each time, they can skip at most one level, and level 1 and level 9 are mandatory. Before the winter of 2022, a group of students became magicians, but the courses they studied were all different. The maximum number of students in this group is $\qquad$ people.
$34$