There is a rectangular cornfield with a length of 13 meters and a width of 7 meters. A fence is built around the cornfield. How long is the fence?
有一块长方形的玉米地，长是13米，宽是7米，在这块玉米地的四周围上篱笆，篱笆长多少米？
<gadget id="calculator">13 + 7</gadget>
<output>20</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<result>40</result>