13 chickens are lined up, and there is a big rooster among them. There are 8 chickens in front of it. How many chickens are behind it?
13只鸡排成一队，其中有1只大公鸡，它的前面有8只鸡，它的后面有几只鸡？
<gadget id="calculator">8 + 1</gadget>
<output>9</output>

<gadget id="calculator">13 - 9</gadget>
<output>4</output>

<result>4</result>