The outer diameter of a ring is 36 cm and the inner diameter is 30 cm. What is the area of the ring?
一个圆环的外直径是36厘米，内直径是30厘米，这个环形的面积是多少．
<gadget id="calculator">36 / 2</gadget>
<output>18</output>

<gadget id="calculator">18 ** 2</gadget>
<output>324</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">15 ** 2</gadget>
<output>225</output>

<gadget id="calculator">324 - 225</gadget>
<output>99</output>

<gadget id="calculator">3.14 * 99</gadget>
<output>310.86</output>

<result>310.86</result>