
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
Let  $\mathcal{A}$  be the set of finite sequences of positive integers  $a_1,a_2,\dots,a_k$  such that  $|a_n-a_{n-1}|=a_{n-2}$  for all  $3\leqslant n\leqslant k$ . If  $a_1=a_2=1$ , and  $k=18$ , determine the number of elements of  $\mathcal{A}$ .

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