circle has an inner diameter of 4 meters and an outer diameter of 10 meters. What is its area in square meters.
一个环形，内圆半径是4米，外圆直径是10米．它的面积是多少平方米．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">25 - 16</gadget>
<output>9</output>

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

<result>28.26</result>