program pascal;
var f,g:text;
    n,i,j:longint; nr:int64;
    x:array[97..122] of integer;
    c:char;

  procedure citire;
  begin
   readln(f,n);
   for i:=1 to n do
      begin
         read(f,c);
         inc(x[ord(c)]);
         for j:=ord(c)+1 to 122 do nr:=nr+x[j];
      end;
  end;


begin
 assign(f,'litere.in'); reset(f);
 assign(g,'litere.out'); rewrite(g);
  nr:=0;
  for j:=97 to 122 do x[j]:=0;
  citire;
  write(g,nr);
 close(f);
 close(g);
end.