Example of Hausdorff dimension of product not equal to sum

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If $A\subset\mathbb{R}^n$ and $B\subset\mathbb{R}^m$, then we have the Hausdorff dimension of the product $A\times B$ is greater than or equal to the sum of the Hausdorff dimensions of $A$ and $B$.

What is an example where the inequality is strict?