Longest Substring with At Most K Distinct Characters

You are given a string s and an integer k. Your task is to find the length of the longest substring of s that contains at most k distinct characters.

If k is negative, the program should output the error message: k must be a non-negative integer.

For example, when s = "eceba" and k = 2, one optimal substring is "ece" with a length of 3.

inputFormat

The input consists of two lines:

1. The first line is a string s (which may be empty).
2. The second line is an integer k.

outputFormat

Output a single line representing the length of the longest substring of s containing at most k distinct characters. If k is negative, output k must be a non-negative integer.

eceba
2

3