#include <iostream>
#include <string>
using namespace std;

int main ()
{
    freopen("ordine.in","r",stdin);
    char s[1000000]; long long i,j,temp; char min, so[1000000];
    j=0;
    gets(s);
    fclose(stdin);
    for(j=0; j<strlen(s); j++)
    {
     min=122;
     for(i=0; i<strlen(s); i++)
        if((int)s[i]<(int)min && s[i]!=so[j-1] && s[i]!='A') 
         {
           min=s[i];
           temp=i;
         }
     so[j]=min;
     s[temp]='A';
     }
    freopen("ordine.out","w",stdout);
    cout<<so; 
    fclose(stdout);
    return 0;
}