A calf is tied to a grass field with a rope 4 meters long. What is the maximum area that the calf can eat grass?
在草地上用4米长的绳子拴一头小牛，小牛能吃到草的最大面积是多少．
<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

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

<result>50.24</result>