Longest Substring with K Distinct Characters

Given a string S and an integer K, find the longest substring of S that contains exactly K distinct characters.

This problem can be formally represented as follows: find a substring S' of S such that:

$|S'|$ is maximized and $\text{distinct}(S') = K$.

If no such substring exists, output an empty string.

inputFormat

The input is read from standard input (stdin) and consists of two lines:

• The first line contains the string S.
• The second line contains the integer K.

outputFormat

Print to standard output (stdout) the longest substring of S that contains exactly K distinct characters. If there is no such substring, print an empty string.

## sample

abcba
2

bcb

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