
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
The sequence $\left\{a_{n}\right\}$ satisfies:
$$
a_{1}=2, \quad a_{n}+a_{n+1}=1 \quad (\text{where } n \in \mathbf{Z}_{+}).
$$

Let $S_{n}$ be the sum of the first $n$ terms of the sequence $\left\{a_{n}\right\}$. Then find the value of
$$
S_{2017} - 2 S_{2018} + S_{2019}.
$$

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