Let $△PQR$ be an isosceles triangle with $PQ=QR$ and $\angle PQR=100^{\circ }$. Construct the circle with diameter $\overline{PR}$, and let $S$ and $T$ be the other intersection points of the circle with the sides $\overline{PQ}$ and $\overline{QR}$, respectively. Let $U$ be the intersection of the diagonals of the quadrilateral $PRST$. What is the degree measure of $\angle PUR$?