Find shortest substring with n unique letters

Problem

You are given a string s consisting of lowercase English letters and an integer n (1 ≤ n ≤ 26).

Find the length of the shortest contiguous substring of s that contains exactly n distinct characters.

If no such substring exists, return -1.

Example

• Input: s = "aabcbcdbca" , n = 3
• One shortest substring with exactly 3 distinct characters is "bca" (length 3), so the output should be 3 .