Squares within squares

Consider a $3\times3$ grid of $1\times1$ squares. Now consider two other grid-squares, the same size as the previous ones, being placed randomly on the $3\times3$ grid. The squares must be entirely on the grid, are allowed to overlap over the interior grid lines, and must have each side of the square parallel to a grid line. If the chance that the squares do not overlap is $\frac{a}{b}$ for coprime positive integers $a$ and $b$ , find $a + b$ .