var a:array['a'..'z'] of longint;
    nr,i,n:longint;
    j,c:char;
begin
assign(input,'litere.in');reset(input);
assign(output,'litere.out');rewrite(output);
readln(n);
for i:=1 to n do
    begin
    read(c);
    for j:='a' to 'z' do
    begin
        if (c<>#26)and(c<j) then
        nr:=nr+a[j];
    end;
    inc(a[c]);
    end;
writeln(nr);
close(output);
end.