A semi-circular chicken pen is surrounded by a wall with a fence. The radius of this semi-circle is 4 meters. How long is the fence required to enclose a double layer?
一个一面靠墙用篱笆围成的半圆形养鸡圈，这个半圆的半径是4米，要围双层需要篱笆长多少米？
<gadget id="calculator">3.14 * 4 * 2</gadget>
<output>25.12</output>

<result>25.12</result>