The farmer's uncle fixed one end of the rope for tying the cattle to the center of the grass, and the rope was 5 meters long. Do the math, what is the maximum area that this cow can graze on?
农民伯伯把栓牛绳子的一头固定在草地的中央，绳子长5米．算一算，这头牛能吃草的最大面积是多少？
<gadget id="calculator">3.14 * 5 * 5</gadget>
<output>78.5</output>

<result>78.5</result>