A ring has an inner diameter of 8 cm and an outer diameter of 18 cm. What is the area of the ring?
一个圆环的内直径8cm，外直径18cm，这个圆环的面积是多少？
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">9 ** 2</gadget>
<output>81</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</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>