If the lengths of two adjacent sides of a parallelogram are 8 and 5 cm respectively, what is its perimeter in cm?
平行四边形中相邻两边分别长8里面和5厘米，它的周长是多少厘米．
<gadget id="calculator">8 + 5</gadget>
<output>13</output>

<gadget id="calculator">13 * 2</gadget>
<output>26</output>

<result>26</result>