Make 20 knots of bottom surface radius be 5 centimeters, be long the cylindrical iron sheet ventilation pipe of 40 centimeters, how much area iron sheet will be used at least?
制20节底面半径为5厘米、长为40厘米的圆柱形铁皮通风管，至少要用多大面积的铁皮？
<gadget id="calculator">3.14 * 5 * 2 * 40</gadget>
<output>1_256</output>

<gadget id="calculator">1_256 * 20</gadget>
<output>25_120</output>

<result>25_120</result>