#include <iostream>

 #include <stdio.h>

 #include <string.h>

 using namespace std;

 const int MAXN = 1e5+;

 int tag[MAXN][], vis[], indx = ;

 char q[];

 int main(void){

     string s;

     cin >> s;

     for(int i = s.size() - ; i >= ; i--){

         for(int j = ; j < ; j++){

             if(j == s[i] - 'a') tag[i][j] = tag[i + ][j] + ;

             else tag[i][j] = tag[i + ][j];

         }

     }

     for(int i = ; i < s.size(); i++){

         if(!vis[s[i] - 'a']){

             while(indx > ){

                 int cnt = q[indx] - 'a';

                 if(cnt < s[i] - 'a' || tag[i + ][cnt] == ) break;

                 --indx;

                 vis[cnt] = ;

             }

             q[++indx] = s[i];

             vis[s[i] - 'a'] = ;

         }

     }

     for(int i = ; i <= indx; i++){

         printf("%c", q[i]);

     }

     puts("");

     return ;

 }