MathNet 2001 — Part 01 Question 36

In the regular 18-gon $A_1 A_2 \ldots A_{18}$ with the circumcenter $M$, $P$ is the intersection of $A_1A_7$ with $MA_2$ and $Q$ is the intersection of $A_2A_{13}$ with $MA_1$. Calculate the angle $\angle MPQ$.