
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
A certain bookstore has two branches that together have 5000 copies of a certain book. If 400 copies of this book are transferred from Branch A to Branch B, then the number of copies in Branch B is still 400 less than half the number of copies in Branch A. Find the original difference in the number of copies of this book between the two branches.

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