A cube with a side length of 3 cm, after digging a small cube with a side length of 1 cm from the top, bottom, left and right, and front and back, a penetrating hollow cube toy is formed. What is the volume of this toy?
一个棱长为3cm的正方体，从上下、左右、前后分别挖一个棱长为1cm的小正方体后，形成一个穿透的中空正方体玩具，这个玩具的体积是多少．
<gadget id="calculator">3 * 3 * 3</gadget>
<output>27</output>

<gadget id="calculator">1 * 1 * 3 * 3</gadget>
<output>9</output>

<gadget id="calculator">27 - 9</gadget>
<output>18</output>

<gadget id="calculator">1 * 1 * 1 * 2</gadget>
<output>2</output>

<gadget id="calculator">18 + 2</gadget>
<output>20</output>

<result>20</result>