program alex;
var f:text;
    p1,p2,j,d:integer;
    i,a:char;
    c:array['a'..'z']of longint;
    b:array[1..1000000]of char;
begin
assign(f,'ordine.in');reset(f);
d:=0;
while not eof(f) do
      begin
      read(f,a);
      c[a]:=c[a]+1;
      d:=d+1;
      end;
close(f);
p1:=1;
p2:=2;
for i:='a' to 'z' do
    if c[i]<>0 then begin
                    if p1<p2 then begin
                                  for j:=1 to c[i] do
                                      begin
                                      b[p1]:=i;
                                      p1:=p1+2;
                                      end;
                                  end
                              else begin
                                   for j:=1 to c[i] do
                                       begin
                                       b[p2]:=i;
                                       p2:=p2+2;
                                       end;
                                   end;
                    end;
assign(f,'ordine.out');rewrite(f);
for j:=1 to d do
    write(f,b[j]);
close(f);
end.