To make a chimney with a bottom diameter of 10 decimeters and a length of 4 meters, at least how many square decimeters of iron sheets are needed.
做一节底面直径为10分米，长4米的烟筒，至少需要多少平方分米铁片．
<gadget id="calculator">4 * 10</gadget>
<output>40</output>

<gadget id="calculator">3.14 * 10 * 40</gadget>
<output>1_256</output>

<result>1_256</result>