
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
Calculate the following sum: 
\(1-2-3+4+5-6-7+8+9-10-11+\cdots + 1998+1999-2000\).
A) 450000
B) 400000
C) 350000
D) 300000

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