$PQRS$ is a isosceles trapezoid. the parallel bases $\overline {PQ}$ and $\overline {RS}$ have lengths $500$ and $650$ respectly. $PS=QR=333$. The angle bisectors of $\angle P$ and $\angle S$ meet at $X$ and the angle bisectors of $\angle Q$ and $\angle R$ meet at $Y$. Find $XY$.