Cow and Message

Bessie the cow has just intercepted a text that Farmer John sent to Burger Queen! However, Bessie is sure that there is a secret message hidden inside.

The text is a string $$$s$$$ of lowercase Latin letters. She considers a string $$$t$$$ as hidden in string $$$s$$$ if $$$t$$$ exists as a subsequence of $$$s$$$ whose indices form an arithmetic progression. For example, the string aab is hidden in string aaabb because it occurs at indices $$$1$$$, $$$3$$$, and $$$5$$$, which form an arithmetic progression with a common difference of $$$2$$$. Bessie thinks that any hidden string that occurs the most times is the secret message. Two occurrences of a subsequence of $$$S$$$ are distinct if the sets of indices are different. Help her find the number of occurrences of the secret message!

For example, in the string aaabb, a is hidden $$$3$$$ times, b is hidden $$$2$$$ times, ab is hidden $$$6$$$ times, aa is hidden $$$3$$$ times, bb is hidden $$$1$$$ time, aab is hidden $$$2$$$ times, aaa is hidden $$$1$$$ time, abb is hidden $$$1$$$ time, aaab is hidden $$$1$$$ time, aabb is hidden $$$1$$$ time, and aaabb is hidden $$$1$$$ time. The number of occurrences of the secret message is $$$6$$$.