Count Substrings

Let's call a string $X$ *interesting* if it satisfies the following conditions: - $X$ contains at least $K$ pairwise distinct characters - for any two characters $c_1$ and $c_2$ which appear in $X$, the frequency (number of occurrences) of $c_1$ in $X$ is equal to the frequency of $c_2$ in $X$ You are given a string $S$. Compute the number of interesting substrings of $S$. Two substrings are cons