A circular iron ring has an outer diameter of 40 cm and an inner diameter of 20 cm. How much iron sheet is needed to make such an iron ring?
一个圆形的铁环，外直径是40厘米，内直径是20厘米，做这样一个铁环需要用多大的铁皮？
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

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

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

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

<gadget id="calculator">400 - 100</gadget>
<output>300</output>

<gadget id="calculator">3.14 * 300</gadget>
<output>942</output>

<result>942</result>