What is the area of a circle with an outer diameter of 6dm and an inner diameter of 4dm?
外圆直径是6dm，内圆直径是4dm的圆环面积是多少．
<gadget id="calculator">6 / 2</gadget>
<output>3</output>

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

<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>