var n,i,nr:integer;
    v:array [1..10000] of char;
    c:array ['a'..'z'] of integer;
    j:char;
begin
 assign(input,'litere.in'); reset(input);
 assign(output,'litere.out'); rewrite(output);
  readln(n);
  for i:=1 to n do read(v[i]);
  for i:=1 to n do begin
   inc(c[v[i]]);
   for j:=v[i] to 'z' do
    if j<>v[i] then inc(nr,c[j]);
  end;
  writeln(nr);
 close(input); close(output);
end.