square iron plate with a circumference of 40 decimeters, cut out the largest circle, what is the area of the circle?
一块周长是40分米的正方形铁板，剪下一个最大的圆，圆的面积是多少．
<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">10 / 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>