If the outer radius of a circle is 4 decimeters and the inner radius is 2 decimeters, what is the area of the circle?
一个圆环外圆半径是4分米，内圆半径是2分米，圆环的面积是多少．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

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

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">3.14 * 12</gadget>
<output>37.68</output>

<result>37.68</result>