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 \(a + b\).

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