A semi-circular chicken farm with a diameter of 8 meters on one side of the wall, how many meters of fence does it need to be fenced around (the side next to the wall is not fenced).
一个一面靠墙的直径为8米的半圆形养鸡场，现在要把周围用篱笆围起来(靠墙的一面不围)需要长多少米的篱笆．
<gadget id="calculator">3.14 / 2</gadget>
<output>1.57</output>

<gadget id="calculator">8 * 1.57</gadget>
<output>12.56</output>

<result>12.56</result>