#include<stdio.h>
#include<string.h>
int b[27],k;
void solve()
{
     char a[10000],an;
     int n,i;
     gets(a);
     n=strlen(a);
     for(i=0;i<n;i++)
              b[a[i]-'a']++;
              for(k=0;k<n;k++)
              {
                               for(i=0;i<=n;i++)
                               {
                                                if(b[i]!=0 && char(i+'a')!=an)
                                                {
                                                           an=char(i+'a');
                                                           b[i]--;
                                                           printf("%c",(char)an);
                                                           }}}}
                                                           
int main ()
{
    freopen("ordine.in","r",stdin);
    freopen("ordine.out","w",stdout);
    solve();
    return 0;
}