ring has an outer diameter of 40 cm and an inner diameter of 20 cm, what is the area of the ring?
一个圆环，外圆直径是40厘米，内圆直径是20厘米，则此圆环的面积是多少．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 ** 2</gadget>
<output>400</output>

<gadget id="calculator">3.14 * 400</gadget>
<output>1_256</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">3.14 * 100</gadget>
<output>314</output>

<gadget id="calculator">1_256 - 314</gadget>
<output>942</output>

<result>942</result>