The radius of the cross-section of the trunk is 20 cm, and the thickness of the bark is 2 cm. What is the cross-sectional area of the trunk that can be used as timber?
树干横截面的半径是20厘米，树皮厚2厘米，树干可用作木料的横截面面积是多少平方厘米？
<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<gadget id="calculator">18 ** 2</gadget>
<output>324</output>

<gadget id="calculator">3.14 * 324</gadget>
<output>1_017.36</output>

<result>1_017.36</result>