A cylindrical pool has an outer diameter of 5 meters, an inner diameter of 4 meters, and a height of 1 meter. What is its volume?
一个圆柱形水池，外直径5米，内直径4米，高1米，它的容积是多少？
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<gadget id="calculator">3.14 * 4 * 1</gadget>
<output>12.56</output>

<result>12.56</result>