The Strongest String - 1
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## Ad-Hoc, Data Structures, Implementation

Problem
Editorial
Analytics

Maga and Alex are good at string manipulation problems. Just now they have faced a problem related to string. But it is not a standard string problem. They have no idea to solve it. They need your help.

A string is called unique if all characters of string are distinct.

String $s_1$ is called subsequence of string $s_2$ if $s_1$ can be produced from $s_2$ by removing some characters of $s_2$.

String $s_1$ is stronger than $s_2$ if $s_1$ is lexicographically greater than $s_2$.

You are given a string. Your task is to find the strongest unique string which is subsequence of given string.

Input:

first line contains length of string.
second line contains the string.

Output:

Output the strongest unique string which is subsequence of given string.

Constraints:

$1 \le |S| \le 100000$

All letters are lowercase English letters.

SAMPLE INPUT
5
abvzx

SAMPLE OUTPUT
zx

Explanation

Select all subsequence of the string and sort them in ascending order. The greatest of all is zx.

Time Limit: 1.0 sec(s) for each input file.
Memory Limit: 64 MB
Source Limit: 1024 KB

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