Groups and the Group Axioms · Easy problem

Let $C=\begin{pmatrix}1&2\\0&1\end{pmatrix}$ and $D=\begin{pmatrix}1&0\\3&1\end{pmatrix}$ under matrix multiplication. Using the law $(CD)^{-1}=D^{-1}C^{-1}$, find the sum of the four entries of $(CD)^{-1}$.