var ap:array['a'..'z'] of longint;
    n,i,k:longint;
    f,g:text;
    c,ch:char;
begin
assign(f,'litere.in');
assign(g,'litere.out');
reset(f);
rewrite(g);
readln(f,n);
for i:=1 to n do begin
                 read(f,c);
                 ap[c]:=ap[c]+1;
                 for ch:=succ(c) to 'z' do
                  if ap[ch]>0 then k:=k+Ap[ch];
                 end;
write(g,k);
close(f);
close(g);
end.