A cylindrical ventilation pipe, the diameter of its bottom surface is 4 decimeters, how many square decimeters of iron sheets are needed to make such a 5-meter ventilation pipe?
一种圆柱形通风管，它的底面直径是4分米，用铁皮制作5米的这种通风管，需要铁皮多少平方分米？
<gadget id="calculator">5 * 10</gadget>
<output>50</output>

<gadget id="calculator">3.14 * 4 * 50</gadget>
<output>628</output>

<result>628</result>