The bottom diameter of a cylindrical oil drum without a cover is 40 cm, and the height is 20 cm. How many square centimeters of iron sheet are needed to make this cylindrical oil drum?
一个无盖的圆柱形油桶的底面直径是40厘米，高是20厘米，做这个圆柱形油桶至少需要铁皮多少平方厘米．
<gadget id="calculator">3.14 * 40 * 20</gadget>
<output>2_512</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 ** 2</gadget>
<output>400</output>

<gadget id="calculator">3.14 * 400</gadget>
<output>1_256</output>

<gadget id="calculator">2_512 + 1_256</gadget>
<output>3_768</output>

<result>3_768</result>