var v:array['a'..'z']of integer;
    c,a:char;f,g:text;
    n,i:integer;nr:longint;
begin
assign(f,'litere.in');reset(f);
assign(g,'litere.out');rewrite(g);
readln(f,n);nr:=0;
for i:=1 to n do begin
         read(f,c);inc(v[c]);
         c:=succ(c);
         for a:=c to 'z' do if v[a]<>0 then inc(nr,v[a]);
         end;
writeln(g,nr);
close(g);
end.