A cow is tied in the middle of a meadow with a rope length of 5 meters. What is the maximum area of grass that the cow can eat?
在一片草地正中拴着一头牛，绳长5米，这头牛最多可吃到的青草的面积多少？
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

<result>78.5</result>