A       B
+-----+
|        |
|        |
+-----+
D       C

Precisely one stone is situated initially at each of the four vertices of the square ABCD. It is permissible to change the number of stones according to the following rule:

Any move consists of taking n stones away from any vertex and adding 2n stones to either adjacent vertex.

Prove that it is not possible to get 1989, 1988, 1990 and 1989 stones respectively at the vertices A, B, C and D after a finite number of moves.