program stefi;
var f,g:text;
    a:array['a'..'z']of longint;
    b:array[1..1000]of char;
    j,c:char;
    n,i,q:longint;
begin
     assign(f,'ordine.in');
     reset(f);
     while not(eoln(f))do
     begin
          read(f,c);
          inc(a[c]);
          inc(n);
     end;
     close(f);
     assign(g,'ordine.out');
     rewrite(g);
     for i:=1 to n do
     begin
          for j:='a'to'z' do
          begin
               if (b[q]<>j)and(a[j]<>0) then
               begin
                    inc(q);
                    b[q]:=chr(ord(j));
                    dec(a[j]);
                    write(g,j);
                    break;
               end;
          end;
     end;
     close(g);
end.