A circular piece of iron has an outer radius of 0.5 meters and an inner radius of 0.4 meters. What is its area.
一块环形铁片，外圆半径是0.5米，内圆半径是0.4米．它的面积是多少．
<gadget id="calculator">0.5 ** 2</gadget>
<output>0.25</output>

<gadget id="calculator">3.14 * 0.25</gadget>
<output>0.785</output>

<gadget id="calculator">0.4 ** 2</gadget>
<output>0.16</output>

<gadget id="calculator">3.14 * 0.16</gadget>
<output>0.5024</output>

<gadget id="calculator">0.785 - 0.5024</gadget>
<output>0.2826</output>

<result>0.2826</result>