Longest Substring with Exactly K Distinct Characters

Given a string s and an integer k, your task is to find the length of the longest contiguous substring of s that contains exactly k distinct characters.

For example, consider the substring s[i...j], if there are exactly k distinct characters, then the length of the substring is j-i+1. You need to output the maximum such length. If no valid substring exists, output 0.

Note that by "exactly k distinct characters" we mean that the substring should have no more and no less than k unique characters. Formally, let the number of distinct characters in the substring be \(d\). The substring is valid if and only if \(d = k\).

inputFormat

The input consists of two lines:

1. The first line contains the string s composed of lowercase English letters.
2. The second line contains an integer k, where k \ge 0.

outputFormat

Output a single integer denoting the length of the longest substring of s that contains exactly k distinct characters. If no such substring exists, output 0.

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