How many square centimeters of iron sheet should be used to make a cylindrical ventilation pipe with a base radius of 12cm and a length of 20cm?
做一个底面半径为12cm，长为20cm的圆柱形通风管，至少要用多少平方厘米的铁皮？
<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">3.14 * 24 * 20</gadget>
<output>1_507.2</output>

<result>1_507.2</result>