If a cow is tied to a tree in the grass with a rope 4 meters long, what is the maximum area of grass that the cow can eat?
把一头牛用4米长的绳子拴在草地中的一棵树上，牛所能吃到草的最大面积是多少平方米？
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<result>50.24</result>