To make a cylindrical bucket, the radius of the bottom surface is 20cm, and the height is 50cm. How many square centimeters of iron sheet are needed at least?
做一个圆柱形水桶，底面半径是20cm，高是50cm，至少需要铁皮多少平方厘米？
<gadget id="calculator">20 ** 2</gadget>
<output>400</output>

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

<gadget id="calculator">3.14 * 20 * 2 * 50</gadget>
<output>6_280</output>

<gadget id="calculator">1_256 + 6_280</gadget>
<output>7_536</output>

<result>7_536</result>