Intermediate Mathematical Challenge 2008 Q21

In triangle $PQR$, $\angle QPR=\alpha^{\circ}$ and $\angle PQR=\beta^{\circ}$, where $\alpha < \beta$. The line $RM$ bisects $\angle PRQ$ and $RN$ is the perpendicular from $R$ to the line $PQ$.

What is the size, in degrees of $\angle MRN$?