The outer radius of a ring is 9 decimeters and the inner radius is 4 decimeters. Find the area of the ring.
一个环形的外圆半径是9分米，内圆半径4分米，求环形的面积．
<gadget id="calculator">9 ** 2</gadget>
<output>81</output>

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

<gadget id="calculator">81 - 16</gadget>
<output>65</output>

<gadget id="calculator">3.14 * 65</gadget>
<output>204.1</output>

<result>204.1</result>