To make a cylindrical ventilation pipe with a bottom diameter of 20 cm and a length of 50 cm, how much iron sheet should be used at least?
制作一个底面直径20厘米，长50厘米的圆柱形通风管，至少要用多少铁皮？
<gadget id="calculator">3.14 * 20 * 50</gadget>
<output>3_140</output>

<result>3_140</result>