How many square decimeters of iron sheet are needed to make a cylindrical ventilation pipe with a length of 1 meter and a circumference of the bottom surface of 2 decimeters? (pipe wall thickness is ignored)
做一根长1米，底面周长是2分米的圆柱形通风管，需要铁皮多少平方分米？(管壁厚度忽略不计)
<gadget id="calculator">1 * 10 * 2</gadget>
<output>20</output>

<result>20</result>