Compute the determinant of the block diagonal matrix: $M = \begin{pmatrix} A & O \\ O & B \end{pmatrix}$ where $A = \begin{pmatrix} 3 & 1 \\ 2 & 4 \end{pmatrix}$ and $B = \begin{pmatrix} -1 & 5 \\ 2 & 3 \end{pmatrix}$ (with $O$ representing zero blocks).