
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given the function $$f(x)= \begin{cases} \sqrt {x}+3, & x\geq0 \\ ax+b, & x<0\end{cases}$$ satisfies the condition: $y=f(x)$ is a monotonic function on $\mathbb{R}$ and $f(a)=-f(b)=4$, then the value of $f(-1)$ is \_\_\_\_\_\_.

Present the answer in LaTex format: \boxed{Your answer}
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