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