58. As shown in the figure, there are $a$ holes on a circle ($a$ is a two-digit number). Xiao Ming, like playing checkers, starts from hole $A$ and jumps in a counterclockwise direction, hoping to jump back to hole $A$ after one round. He first tries to jump every 2 holes, but ends up at hole $B$. Then he tries to jump every 4 holes, and still ends up at hole $B$. Finally, he jumps every 6 holes and successfully returns to hole $A$. Therefore, $a=$ $\qquad$ .
Answer: 91