Use a piece of square paper with a side length of 20 cm to enclose the side of a cylinder. What is the side area of the cylinder in square centimeters?
用一张边长是20厘米的正方形纸围成一个圆柱体的侧面，这个圆柱体的侧面积是多少平方厘米．
<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<result>400</result>