Given a string s and an integer k, return the total number of substrings of s where at least one character appears at least k times.

A substring is a contiguous non empty sequence of characters within a string.

Constraints:

• $1 \leq$ s.length $\leq 3000$

• $1 \leq$ k $\leq$ s.length

• s consists only of lowercase English letters.

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