CN-SMO 2005 Problem 5

Line $l$ tangents unit circle $S$ in point $P$ . Point $A$ and circle $S$ are on the same side of $l$ , and the distance from $A$ to $l$ is $h$ ( $h > 2$ ). Two tangents of circle $S$ are drawn from $A$ , and intersect line $l$ at points $B$ and $C$ respectively.

Find the value of $PB \cdot PC$ .