There is a tree in the middle of the grass, and a cow is tied to the tree trunk by a 5-meter-long rope. What is the area of grass that the cow can eat?
草地中央有一棵树，一头牛被5米长的绳子拴在树干上，这头牛能吃到草的面积是多少．
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<result>78.5</result>