On the grass, a 5-meter-long rope is tied to a cow. What is the maximum area of grass that the cow can eat?
草地上，一根5米长的绳子拴着一头牛．这头牛最多能吃到多大面积的青草？
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<result>78.5</result>