A ring has an outer diameter of 30 cm and an inner diameter of 20 cm. What is the area of the ring in square centimeters?
一个环形，外圆直径是30厘米，内圆直径是20厘米，这个环形的面积是多少平方厘米？
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

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

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">225 - 100</gadget>
<output>125</output>

<gadget id="calculator">3.14 * 125</gadget>
<output>392.5</output>

<result>392.5</result>