CONO-SUR 2012 Problem 2

In a square $ABCD$ , let $P$ be a point in the side $CD$ , different from $C$ and $D$ . In the triangle $ABP$ , the altitudes $AQ$ and $BR$ are drawn, and let $S$ be the intersection point of lines $CQ$ and $DR$ . Show that $\angle ASB=90$ .