Var f,g:Text;
    x,s,p:String;
    i,j,a:Longint;

Begin

Assign(f,'ordine.in'); ReSet(f);
Assign(g,'ordine.out'); ReWrite(g);

readln(f,s);
p:='';

For i:=1 To Length(s) Do Begin
    x:=s[i];
    a:=Length(p);
    If p='' Then p:=p+x;
    For j:=1 To Length(p) Do
        If (x<p[j])and(x<>p[j])and(x<>p[j-1]) Then Begin
           Insert(x,p,j);
           Break;
           End;
    If (j=Length(p))and(a=length(p)) Then p:=p+x;
    End;

WriteLn(g,p);

Close(f);
Close(g);

End.