NMC 1999 Problem 1

The function $f$ is defined for non-negative integers and satisfies the condition

$$f(n)= \begin{cases}f(f(n+11)), & \text { if } n \leq 1999 \\ n-5, & \text { if } n>1999\end{cases}$$

Find all solutions of the equation $f(n)=1999$.