var f,g:text;
    p,n,c:char;
    a:array['a'..'z'] of longint;
begin
assign(f,'ordine.in');reset(f);
assign(g,'ordine.out');rewrite(g);
while not eof(f) do begin
 while not eoln(f) do begin
  read(f,c);
  inc(a[c]);
 end;
 readln(f);
end;
p:='a';
while a[p]<>0 do p:=chr(ord(p)+1);
n:=chr(ord(p)+1);
while a[p]<>0 do n:=chr(ord(n)+1);
while ord(p)<=ord('z') do begin
 write(g,p);
 dec(a[p]);
 write(g,n);
 dec(a[n]);
 if a[p]=0 then begin
  p:=n;
  n:=chr(ord(p)+1);
  while a[p]<>0 do n:=chr(ord(n)+1);
 end;
end;
close(f);
close(g);
end.