20. In a $4 \times 4$ grid, there are 16 chess pieces, each with one black side and one white side. The rule is: flipping the 4 pieces in a $2 \times 2$ subgrid once is called “one operation”. Initially, all 16 pieces have their black sides up, as shown in the left figure. The minimum number of operations required to achieve the situation shown in the right figure is $\qquad$.
$6$