There is a flock of chickens and rabbits. The total number of legs is 18 more than twice the total number of heads. How many rabbits are there?
有一群鸡和兔，腿的总数比头的总数的2倍多18只，兔有几只？
<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<result>9</result>