For the first round, I needed to complete the online assessment. I did not proceed further, so I cannot describe the next round.

A string A contains k lowercase letters and can form multiple subsequences (not necessarily contiguous). If each lowercase letter appears with the same frequency in a subsequence as in the original string, then this subsequence is considered a perfect subsequence.

Given a string A, determine the number of its perfect subsequences.

For example, in the string "abca", the subsequences "a", "b", "c", "a", "ab", "bc", "ca", "aa", etc., are all valid.