28. As shown in the figure, robots $A$ and $B$ start from point A at the same time, moving in opposite directions on a circular path with a diameter of an integer number of meters. They meet 8 times in 18 minutes; if $A$'s speed is increased by 6 meters per minute, then $A$ and $B$ meet 10 times in 18 minutes. The minimum diameter of the circular path is $\qquad$ meters. $(\pi$ is taken as 3.14)
$12$