
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 sequence $\{a_n\}(n\in\mathbb{N^*})$ satisfies: $a_n= \begin{cases} n &(n=1,2,3,4,5,6) \\ -a_{n-3} &(n\geqslant 7\text{ and }n\in\mathbb{N^*}) \end{cases},$ find $a_{2015}$ and $S_{2015}$.

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