If the radius of a circle increases from 2 cm to 4 cm, how many cm does its circumference increase.
如果一个圆的半径由2厘米增加到4厘米，它的周长就要增加多少厘米．
<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">3.14 * 2 * 2</gadget>
<output>12.56</output>

<result>12.56</result>