A circular pond, its circumference is 31.4 meters, find its area.
一个圆形池塘，它的周长是31.4米，求它的面积．
<gadget id="calculator">31.4 / 3.14 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<result>78.5</result>