square is surrounded by an iron wire. The perimeter of this square is 31.4dm. If a circle is formed by this iron wire, what is the area of the circle in square decimeters?
用一根铁丝围成一个正方形，这个正方形的周长是31.4dm，如果用这根铁丝围成一个圆形，这个圆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>