KöMaL 2021 K706

Three numbers $a$, $b$, $c$ are entered (left to right) in the first row of a three column table. The numbers in the second row are $a-b$, $b-c$, $c-a$. In the third row, the numbers are obtained by the same rule from the second row (the same operations carried out with the numbers of the first, second and third fields), and so on. Show that from the fourth row onwards 2021 cannot occur in the table.