Cut out a circle with as large an area as possible from a piece of square paper with a side length of 4 decimeters. What is the area of this circle in square decimeters?
用一张边长是4分米的正方形纸剪一个面积尽可能大的圆，这个圆的面积是多少平方分米．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">3.14 * 4</gadget>
<output>12.56</output>

<result>12.56</result>