Take K of Each Character From Left and Right

Given a string of 'a', 'b', 'c' and integer k, compute the minimum number of characters you must take from the ends to collect at least k of each letter (or -1 if any letter appears < k). Key insight: if totals suffice, this reduces to finding the longest contiguous middle substring whose counts of each letter are ≤ total - k, which can be found with a two-pointer/sliding-window scan.