var v:array[0..400]of longint;
    n,i,j,k:longint;
    c:char;
    f:text;
begin
   assign(f,'litere.in');
   reset(f);
   readln(f,n);
   for i:=1 to n do
   begin
   read(f,c);
   v[ord(c)-96]:=v[ord(c)-96]+1;
   for j:=ord(c)-96+1 to 30 do
   k:=k+v[j];
   end;
   close(f);
   assign(f,'litere.out');
   rewrite(f);
   writeln(f,k);
   close(f);
end.