Use 36 flags to form a solid square, how many chess pieces are there in the outermost layer of the square.
用36颗旗子排成一个实心的方阵，这个方阵的最外层共有多少颗棋子．
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">24 - 4</gadget>
<output>20</output>

<result>20</result>