13. The Youth Palace is recruiting for spring calligraphy, art, and music instrument classes. The calligraphy class has enrolled 29 students, among whom 13 are enrolled in both calligraphy and art, 12 are enrolled in both calligraphy and music instruments, and 5 are enrolled in all three subjects. Therefore, the number of students who only participate in calligraphy is $\qquad$.

$9$