Trees are to be planted on both sides of the road, and 409 trees are to be planted on each side. How many trees are to be planted on both sides?
公路两旁要种树，每旁要种409棵，两旁一共要种多少棵．
<gadget id="calculator">409 * 2</gadget>
<output>818</output>

<result>818</result>