There are nine \( 3 \times 3 \) sub-grids in the \( 5 \times 5 \) grid. If at most 5 out of 9 unit squares are shaded blue in any \( 3 \times 3 \) sub-grid, what is the maximum number of unit squares in the \( 5 \times 5 \) grid that can be shaded blue?

**Note**: An example of how the grid can be shaded is shown to the right.

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