The outer radius of a ring is 5 cm, and the inner diameter is 4 cm. What is the area of the ring?
一个圆环的外圆半径是5厘米，内院直径是4厘米，圆环的面积是多少．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

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

<gadget id="calculator">25 - 4</gadget>
<output>21</output>

<gadget id="calculator">3.14 * 21</gadget>
<output>65.94</output>

<result>65.94</result>