When Xiaoqiang deduced the formula for the area of a circle, he first divided a circular piece of paper into several equal parts, and then assembled them into an approximate rectangle. It is known that the length of the rectangle formed by Xiaoqiang is 6.28 cm. What is the area of the circular paper he used in square centimeters?
小强在推导圆的面积公式时，先将一圆形纸片等分成了若干份，然后拼成一个近似的长方形．已知小强所拼成的长方形的长是6.28厘米，他所用圆形纸片的面积是多少平方厘米．
<gadget id="calculator">6.28 / 3.14</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>