var
	c : array [1..10000] of char;
  f : array ['`'..'z'] of integer; {caracterul mai mic decat 'a'}
  fi, fo : text;
  n, i : integer;
  m : longint;
  l : char;

procedure tipc;
var
	i : integer;
begin
	for i := 1 to n do
  	write(c[i]);
  writeln	
end;
begin
	assign(fi,'litere.in');  reset(fi);
  assign(fo,'litere.out'); rewrite(fo);
  readln(fi,n);
  for i := 1 to n do
  	read(fi,c[i]);
  f[c[1]] := 1;
  for i := 2 to n do
  	begin
      for l := succ(c[i]) to 'z' do
      	inc(m,f[l]);
      inc(f[c[i]]);
    end;
  write (m);
  close(fi); close(fo);	
end.