If a square cardboard with a side length of 4 decimeters is surrounded into a cylindrical paper tube, what is the side area of the paper tube in square decimeters?
把一个边长4分米的正方形纸板，围成一个圆柱形纸筒，这个纸筒的侧面积是多少平方分米．
<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<result>16</result>