Use a fence to surround a circular chicken pen with a radius of 4 meters. How many meters of fence is needed?
用篱笆围一个半径4米的圆形鸡圈，需要篱笆多少米？
<gadget id="calculator">2 * 3.14 * 4</gadget>
<output>25.12</output>

<result>25.12</result>