The perimeter of the rectangle is 24 meters, the width is 4 meters, and the length is in meters.
长方形的周长是24米，宽是4米，长是多少米．
<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<result>8</result>