A ring has an inner radius of 2 cm and an outer radius of 3 cm. What is the area of the ring in square centimetres?
一个环形，内圆半径2厘米，外圆半径3厘米，环形面积是多少平方厘米．
<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

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

<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">3.14 * 5</gadget>
<output>15.7</output>

<result>15.7</result>