var fin, fout:text;
    n, i, k:longint;
    c, aux:char;
    a:array[1..10001] of char;
    sortat:boolean;

begin
   assign(fin, 'litere.in');
   assign(fout, 'litere.out');
   reset(fin);
   rewrite(fout);
   readln(fin, n);
   for i:=1 to n do
      begin
         read(fin, c);
         a[i]:=c;
      end;
   k:=0;
   repeat
      sortat:=true;
      for i:=1 to n-1 do
         if a[i]>a[i+1] then
            begin
               aux:=a[i];
               a[i]:=a[i+1];
               a[i+1]:=aux;
               sortat:=false;
               inc(k);
            end;
   until sortat=true;
   writeln(fout, k);
   close(fin);
   close(fout);
end.