A 4x4 grid has 3 shaded squares. Determine the minimum number of additional squares that need to be shaded to achieve rotational symmetry of order 2.

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Question 6:
To achieve rotational symmetry of order 2, if a square is shaded, the square diagonally opposite to it through the center of the grid must also be shaded.
The original figure has 3 shaded squares. Let's assume the grid coordinates are $(row, column)$ from $(1,1)$ at the top-left to $(4,4)$ at the bottom-right.
The initially shaded squares are:

• $(1,1)$
• $(1,2)$
• $(2,1)$

To make the figure rotationally symmetric of order 2, we need to shade the corresponding squares after a $180^\circ$ rotation.

• For $(1