To make a cylindrical ventilation pipe with a bottom diameter of 4 decimeters and a length of 50 centimeters, at least how many cm**2 of iron sheet should be used.
制作一个底面直径是4分米，长是50厘米的圆柱形通风管，至少要用铁皮多少cm**2．
<gadget id="calculator">4 * 10</gadget>
<output>40</output>

<gadget id="calculator">3.14 * 40 * 50</gadget>
<output>6_280</output>

<result>6_280</result>