9. A staircase has $n$ steps, and each step can cover one or two steps at a time. Let the number of ways to reach the top of the staircase be $f(n)$, then the recurrence relation between $f(n)$, $f(n-1)$, and $f(n-2)$ is

保留源文本的换行和格式，直接输出翻译结果。 

(Note: The last sentence is a note in Chinese and is not part of the translation. It is provided for context.)
9. $f(n)=f(n-1)+f(n-2)$.