Cut a circular piece of paper into two equal semicircles, the circumference of which is increased by 20 cm, what is the area of the circle?
把一张圆形纸片剪成两个相等的半圆，它的周长增加了20厘米，这个圆的面积是多少．
<gadget id="calculator">20 / 2 / 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>