To make a ventilation pipe with a bottom diameter of 20 cm and a length of 40 cm, at least how many square centimeters of iron sheet is needed.
做一节底面直径是20厘米，长40厘米的通风管，至少需要铁皮多少平方厘米．
<gadget id="calculator">3.14 * 20 * 40</gadget>
<output>2_512</output>

<result>2_512</result>