Shortest Unique Characters

Given a string of letters, your task is to construct and return the shortest possible string that contains every unique character from the original string, preserving the order of their first appearance.

For example, if the input string is "abac", the output should be "abc". This problem focuses on string manipulation and maintaining sequence order while eliminating duplicates.

inputFormat

The input consists of a single line containing a string of letters. The string may be empty.

outputFormat

Output the resulting string containing only the unique characters of the input, in the order they first appeared.

abac

abc