Longest Substring with Two Distinct Characters

Given a string s, your task is to find the longest contiguous substring that contains exactly two distinct characters.

The solution must be implemented using a sliding window technique. In case of multiple substrings of maximum length, the function should return the one that appears first.

For example, when s = "eceba", the longest substring with exactly two distinct characters is "ece".

All input will be provided through standard input (stdin) and the answer should be output through standard output (stdout).

inputFormat

The input consists of a single line containing a non-empty string s (which might also be empty in some cases). The string contains only ASCII characters.

outputFormat

Output the longest contiguous substring of s that contains exactly two distinct characters. If the input string is empty, output an empty string.

eceba

ece