Roll a square paper with a side length of 2.5dm into the largest cylinder, what is the lateral area of the cylinder?
把边长2.5dm的正方形纸，卷成一个最大的圆筒，此圆筒的侧面积是多少．
<gadget id="calculator">2.5 * 2.5</gadget>
<output>6.25</output>

<result>6.25</result>