Longest Unique Substring

Given a string \(s\), your task is to find the longest substring that contains only unique characters. A substring is a contiguous sequence of characters within a string. If there are multiple substrings with the maximum length, return the one that appears first in \(s\).

Note: The solution should read input from standard input (stdin) and output the result to standard output (stdout).

Example:

Input: abcabcbb
Output: abc

inputFormat

The input consists of a single line containing the string \(s\). The string can be empty or contain any characters.

outputFormat

Output a single line containing the longest substring of \(s\) that has all unique characters. If the input string is empty, output an empty string.

abcabcbb

abc