Construct a ring with an inner radius of 2 cm and an outer radius of 3 cm. Calculate the area of the circle.
作一个内圆半径为2cm，外圆半径为3cm的一个圆环．计算出圆环的面积．
<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>