There are 18 parrots and 14 cuckoos perched on the 3 trees. There are at least 4 parrots and 1 cuckoo on each tree. If there are no more cuckoos on each tree than parrots, what is the maximum number of birds on a tree?
在3棵树上共栖息着18只鹦鹉和14只杜鹃．每棵树上至少有4只鹦鹉和1只杜鹃．如果每棵树上的杜鹃都不会比鹦鹉多，那么一棵树上最多有多少只鸟．
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">18 - 8</gadget>
<output>10</output>

<gadget id="calculator">10 + 10</gadget>
<output>20</output>

<result>20</result>