
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
The set $S=\{1,2,3,\ldots,20\}$ has a 4-element subset $T=\{a_1,a_2,a_3,a_4\}$, in which the absolute difference between any two elements is not equal to 1. The number of such 4-element subsets $T$ is    ． (Use numbers for the answer)

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