If the outer and inner diameters of a ring are both 4 cm, what is the area of the ring?
一个环形的外圆半径和内圆直径都是4厘米，这个环形的面积是多少．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<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>