Uncle Li used a rope to surround a semicircular vegetable field against the wall. The rope is 31.4 meters long. What is the maximum area of the vegetable field in square meters?
李叔叔用绳子靠墙圈了一块半圆形菜地，绳子长31.4米，菜地的面积最大是多少平方米？
<gadget id="calculator">31.4 * 2</gadget>
<output>62.8</output>

<gadget id="calculator">62.8 / 3.14 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">100 / 2</gadget>
<output>50</output>

<gadget id="calculator">3.14 * 50</gadget>
<output>157</output>

<result>157</result>