Hints:
-  Use 3variable which means first,too and last and
-  do for n downto 1 ...
-  use "1 shr i" to get 2^i..
-  eachtime just swap 2 of (from,temp or too)...

and...
  for i:=n downto 1 do begin
    if p[i]=temp then
      begin writeln(-1); exit; end
    else
      if p[i]=from then
        swap(temp,too)
      else begin
        inc(ans,(1 shl (i-1)));
        swap(from,temp);
      end;
  end;

For more information:
  aidin_n7@hotmail.com

Yeah cool problem IMHO