Longest Substring with Two Distinct Characters

Longest Substring with Two Distinct Characters

You are given a string s. Your task is to find the longest contiguous substring of s that contains exactly two distinct characters. If there are several substrings with the maximum length, output the one which appears first.

Note: A substring is a contiguous sequence of characters within a string. An efficient solution with a time complexity of \( O(n) \) using the sliding-window technique is expected.

Examples:
For s = "abcbbbbcccbdddadacb", the output should be "bcbbbbcccb".
For s = "aabbcc", the output should be "aabb".
For s = "abc", the output should be "ab" because only the first two characters form a valid substring with exactly two distinct characters.

inputFormat

The input is read from stdin and consists of a single line containing the string s.

outputFormat

Output the longest substring of s that contains exactly two distinct characters to stdout.

## sample

abcbbbbcccbdddadacb

bcbbbbcccb