To make a ventilation pipe with a bottom diameter of 20cm and a length of 50cm, how much iron sheet is needed at least in cm**2.
做一节底面直径是20cm，长50cm的通风管，至少需要铁皮多少cm**2．
<gadget id="calculator">3.14 * 20 * 50</gadget>
<output>3_140</output>

<result>3_140</result>