The inner diameter of a ring is 4 cm, the outer diameter is 6 cm, what is the area of the ring in cm**2.
一个圆环内直径为4cm，外直径为6cm，圆环的面积为多少cm**2．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

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

<gadget id="calculator">3.14 * 4</gadget>
<output>12.56</output>

<gadget id="calculator">28.26 - 12.56</gadget>
<output>15.7</output>

<result>15.7</result>