A ring has an inner radius of 4 cm and an outer diameter of 10 cm. Calculate the area of the ring in square centimeters.
一个环形，内圆半径是4厘米，外圆直径是10厘米，计算这个环形的面积是多少平方厘米．
<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>