Find longest substring with at least k repeats

### Longest substring where each character appears at least *k* times You are given: - a string `s` consisting of lowercase English letters - an integer `k` Return the **length of the longest substring** of `s` such that **every distinct character in that substring appears at least `k` times** within the substring. If no such substring exists, return `-1`. #### Notes / assumptions - A substring is contiguous. - If `k <= 1`, the answer is `len(s)`. - You should handle edge cases like empty string, `k > len(s)`, and repeated characters. #### Example - `s = "aaabb", k = 3` → longest valid substring is `"aaa"` → return `3`.