15. Use regular hexagonal tiles of the same size to pave a square in the way shown in the figure. The central regular hexagonal tile is denoted as $A$ and defined as the 1st group; Surround it with six regular hexagonal tiles of the same size, defined as the 2nd group; Pave the outer part of the 2nd group with regular hexagonal tiles of the same size to fill it up, defined as the 3rd group; $\square$ $\qquad$ Following this method, using 2020 tiles starting from the 1st group to pave outward, the maximum number of complete groups that can be paved is $\qquad$.
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