Senior Mathematical Challenge 2015 Q12

A circle touches the sides of triangle $PQR$ at the points $S$, $T$ and $U$ as shown. Also $\angle PQR=\alpha^{\circ}$, $\angle PRQ=\beta^{\circ}$ and $\angle TSU=\gamma^{\circ}$.

Which of the following gives $\gamma$ in terms of $\alpha$ and $\beta$?