Life In 19x19

Hi all!

Playing with IgowinLife, around level 70/80, I found this problem:

Click Here To Show Diagram Code

[go]$$Wc
$$-------------------
$$ . X O . . . . . . |
$$ . X O O . . . X . |
$$ . X X . O O O X . |
$$ . . . X X X X . . |
$$ . . . . . . . . . |[/go]

My solution was:

Click Here To Show Diagram Code

[go]$$Wc
$$-------------------
$$ . X O . 1 5 2 . . |
$$ . X O O . 3 4 X . |
$$ . X X . O O O X . |
$$ . . . X X X X . . |
$$ . . . . . . . . . |[/go]

The program says that it's wrong...or better..it works, but it's a 1/2 point less than the correct solution. I worked a little in finding a solution ending in sente, but i get only "wrong" answer.
It is worth saying that the first move is marked as correct, then Q18 is wrong, as well as Q19 and other obviuos placements.

May you suggest some hints??

Thank you!

Try to remove the black point at S19.

Note, however, that black 4 in your diagram is gote (which is the reason why the correct move is only 1/2 of a point better).