The radius of a wheel is 0.5 meters, how many meters does the wheel move in one revolution?
一个车轮的半径为0.5米，车轮转动一周，大约前进多少米．
<gadget id="calculator">2 * 0.5</gadget>
<output>1</output>

<gadget id="calculator">3.14 * 1</gadget>
<output>3.14</output>

<result>3.14</result>