7. In the cage, there are 2 rabbits and 3 chickens. Count them, the number of heads is less than the number of feet by $\qquad$.

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。 

Note: The blank space represented by $\qquad$ is kept as is in the translation.
$9$