If the circumference of a circle is 31.4 decimeters, what is its area in square decimeters?
一个圆的周长是31.4分米，它的面积是多少平方分米．
<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>