A square iron sheet with a circumference of 80 meters is rolled into a cylinder, what is the surface area of the cylinder in square meters?
把一张周长80米的正方形铁皮卷成一个圆筒，这个圆筒的表面积是多少平方米．
<gadget id="calculator">80 / 4</gadget>
<output>20</output>

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

<result>400</result>