If the radius of a circle increases from 1 meter to 2 meters, how much does the circumference increase in meters (the result is expressed in π).
圆的半径由1米增加到2米，周长增加多少米(结果用π表示)．
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<result>2</result>