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Given a string, find the shortest substring that can be removed to yield a string that contains exactly K different characters.

You are given a string S consisting of N characters and an integer K. You can modify string S by removing any substring of it. A substring is defined as a contiguous segment of a string.

The goal is to find the shortest substring of S that, after removal, leaves S containing exactly K different characters.

Write a function:

class Solution { public int solution(String S, int K); }

that, given a non-empty string S consisting of N characters and an integer K, returns the length of the shortest substring that can be removed. If there is no such substring, your function should return −1.

**Examples:**

1. Given S = "`abaacbca`" and K = 2, your function should return 3. After removing substring "`cbc`", string S will contain exactly two different characters: `a` and `b`.

2. Given S = "`aabcabc`" and K = 1, your function should return 5. After removing "`bcabc`", string S will contain exactly one character: `a`.

3. Given S = "`zaaaa`" and K = 1, your function should return 1. You can remove only one letter: `z`.

4. Given S = "`aaaa`" and K = 2, your function should return −1. There is no such substring of S that, after removal, leaves S containing exactly 2 different characters.

Write an ** efficient** algorithm for the following assumptions:

- N is an integer within the range [1..1,000,000];
- string S is made only of lowercase letters (
a−z);- K is an integer within the range [0..26].

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