
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
Arrange the positive integers in the following number matrix:
\begin{tabular}{lllll}
1 & 2 & 5 & 10 & $\ldots$ \\
4 & 3 & 6 & 11 & $\ldots$ \\
9 & 8 & 7 & 12 & $\ldots$ \\
16 & 15 & 14 & 13 & $\ldots$ \\
$\ldots$ & $\ldots$ & $\ldots$ & $\ldots$ & $\ldots$
\end{tabular}

What is the number in the 21st row and 21st column?

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