var f:text;
    c:char;
    d,e,i,k,n:longint;
    da:boolean;
    a,poz:array[0..26] of longint;
begin
 assign(f,'ordine.in');
 reset(f);
 while not eoln(f) do
  begin
   read(f,c);
   a[ord(c)-96]:=a[ord(c)-96]+1;
   n:=n+1;
  end;
 for i:=1 to 26 do
  if a[i]>0 then
   begin
    k:=k+1;
    poz[k]:=i;
   end;
 close(f);
 d:=1; e:=2;
 while d<=k do
  begin
   if a[poz[d]]>0 then
    begin
     write(chr(poz[d]+96));
     a[poz[d]]:=a[poz[d]]-1;
    end;
   if a[poz[d]]=0 then d:=d+1;
   if a[poz[e]]>0 then
    begin
     write(chr(poz[e]+96));
     a[poz[e]]:=a[poz[e]]-1;
    end;
   if a[poz[e]]=0 then e:=e+1;
   if d=e then d:=d+1;
  end;
end.