program litere{pre ONI 2008 runda2};
var f,g:text;
   c,e:char;
   a:array['a'..'z']of integer;
   i,n:integer;
   s:longint;
begin
assign(f,'litere.in');reset(f);
assign(g,'litere.out');rewrite(g);
for c:='a' to 'z' do
  begin a[c]:=0;  end;
readln(f,n);
for i:=1 to n do
  begin
  read(f,c);
   a[c]:=a[c]+1;
  for e:=chr ( ord(c)+1) to 'z' do
    if a[e]>0 then s:=s+a[e];
 end;
writeln(g,s);
close (f);close(g);
end.