
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
Define "equal sum sequence": In a sequence, if the sum of any two adjacent terms is equal to the same constant, then this sequence is called an equal sum sequence, and this constant is called the common sum of the sequence. It is known that the sequence $\{a_n\}$ is an equal sum sequence, $S_n$ is the sum of its first $n$ terms, and $a_1=2$, the common sum is 5, then $S_9= \boxed{\text{\_\_\_\_\_\_}}$.

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