import java.util.*;
public class Main {
public static String SheldonUniqueString(String str)
{
int[] lastInd=new int[26];
Stack stk=new Stack<>();
for(int i=0;i<str.length();i++){
lastInd[str.charAt(i)-‘a’]=i;
}
for(int i=0;i<str.length();i++)
{
if(!stk.isEmpty())
{
if(stk.contains(str.charAt(i)))
{
continue;
}
while(!stk.isEmpty() && stk.peek()>str.charAt(i) && lastInd[stk.peek()-‘a’]>lastInd[str.charAt(i)-‘a’] )
{
stk.pop();
}
}
stk.push(str.charAt(i));
}
StringBuilder stb=new StringBuilder();
while(!stk.isEmpty()){
stb.append(stk.pop());
}
return stb.reverse().toString();
}
public static void main (String args[]) {
Scanner scn=new Scanner(System.in);
while(scn.hasNext()){
System.out.println(SheldonUniqueString(scn.next()));
}
}
}