Xiao Ming tied a cow to a wooden stake with a 5-meter-long rope. What is the maximum activity area of the cow in square meters?
小明用5米长的绳子将一头牛系在一根木桩上，这头牛的最大活动面积是多少平方米．
<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<result>78.5</result>