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.

Note: A substring is a contiguous sequence of characters within a string. For example, "edu" is a substring of "educative". 

Constraints:

• $1 \leq$ s.length $\leq 3 \times 10^{3}$

• $1 \leq$ k $\leq$ s.length

• s consists only of lowercase English letters.

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