var fi,fo:text;
    n,i,ct:longint;
    lit:array['a'..'z']of integer;
    x,j:char;
begin
  assign(fi,'litere.in'); reset(fi);
  assign(fo,'litere.out'); rewrite(fo);
  readln(fi,n);
  ct:=0;
  for i:=1 to n do
    begin
      read(fi,x);
      for j:=x to 'z' do
         inc(ct,lit[j]);
      dec(ct,lit[x]);
      inc(lit[x]);
    end;
  writeln(fo,ct);
  close(fi);
  close(fo);
end.