
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
It is easy to see that $1991 = 2 \times 4^5 + 0 \times 4^4 + (-1) \times 4^3 + 0 \times 4^2 + 2 \times 4^1 + (-1) \times 1$. The characteristic of this expression is that the sum on the right side involves several multiples of 1, $4^1$, $4^2$, $4^3$, ..., where the multiples are either -1, 0, 1, or 2 times, and the highest power of 4 is less than 0 times. For this reason, we abbreviate this expression as $1991 = (2, 0, -1, 0, 2, -1)_4$. Now, if we need to change the multiples to -2, -1, 0, or 1 times, while other methods and meanings remain unchanged, then the expression should be rewritten as $1991 = (\quad)_4$.

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