var a:array[1..26] of longint;
    v:array[1..1000000] of char;
    i,j,ind,nr:longint;
    c:char;

    f,g:text;

begin
assign(f,'ordine.in');reset(f);
assign(g,'ordine.out');rewrite(g);

nr:=0;
while not eof(f) do
begin
	while not eoln(f) do
	begin
		read(f,c);
                inc(nr);
		inc(a[ord(c)-96]);
	end;
	readln(f);
end;

ind:=1;
for i:=1 to 26 do
begin
	for j:=1 to a[i] do v[ind+2*(j-1)]:=chr(i+96);
	while (ord(v[ind])>=97) and (ord(v[ind])<=123) do inc(ind);
end;

for i:=1 to nr do write(g,v[i]);
writeln(g);
close(g);

end.