
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
Define the sequence \(\left\{a_n\right\}\) as follows:
\[
\begin{aligned}
& a_1 = 1, \\
& a_2 = 3, \\
& a_3 = 5, \\
& a_n = a_{n-1} - a_{n-2} + a_{n-3} \quad \text{for} \; n = 4, 5, \ldots
\end{aligned}
\]

Determine the sum of the first 2015 terms of this sequence, \( S_{2015} \).

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