Grandpa Li built a semi-circular vegetable field with a fence against the wall to cultivate good seeds. The fence is 12.56 meters long. How many square meters is the vegetable field?
李爷爷靠墙用篱笆围了一块半圆形菜地进行良种培植，篱笆长12.56米，菜地面积是多少平方米？
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">3.14 * 8</gadget>
<output>25.12</output>

<result>25.12</result>