
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
An infinite sequence starts with $1$, followed by two $2$s, then three $3$s, then four $4$s, and so on. Denote this sequence as ${a_n}$. If $a_{n-1}=20$ and $a_n=21$, find the value of $n$.

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