A ring has an inner radius of 3 cm and an outer radius of 4 cm. What is the area of the ring?
一个圆环，内圆半径3厘米，外圆半径4厘米，这个圆环的面积是多少．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

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

<gadget id="calculator">50.24 - 28.26</gadget>
<output>21.98</output>

<result>21.98</result>