What is the perimeter of a rectangle with a length of 13 cm and a width of 5 cm?
一个长方形长13厘米，宽5厘米，它的周长是多少．
<gadget id="calculator">13 + 5</gadget>
<output>18</output>

<gadget id="calculator">18 * 2</gadget>
<output>36</output>

<result>36</result>