To make a chimney with a bottom diameter of 2 decimeters and a length of 4 meters, at least how many square decimeters of iron sheet is needed.
要做一个底面直径是2分米，长4米的烟囱，至少需要多少平方分米铁皮．
<gadget id="calculator">4 * 10</gadget>
<output>40</output>

<gadget id="calculator">3.14 * 2 * 40</gadget>
<output>251.2</output>

<result>251.2</result>