(Little Euler seeks the sheepfold). A 100-meter-long fence forms a quadrilateral sheepfold. What is the maximum area of the sheepfold?
(小欧拉智求羊圈)．用100米长的栅栏围成一个四边形的羊圈，羊圈的面积最大是多少？
<gadget id="calculator">100 / 4</gadget>
<output>25</output>

<gadget id="calculator">25 * 25</gadget>
<output>625</output>

<result>625</result>