Groups and the Group Axioms · Medium problem

Let $A=\begin{pmatrix}1&1\\0&1\end{pmatrix}$, $B=\begin{pmatrix}1&0\\1&1\end{pmatrix}$, $D=\begin{pmatrix}1&2\\0&1\end{pmatrix}$. Using $(ABD)^{-1}=D^{-1}B^{-1}A^{-1}$, find the sum of the four entries of $(ABD)^{-1}$.