See on one iteration of "for":
let be x and y on start:x1 and y1.
then after each change it'll be for x1 x2...x3; for y1 y2...y3;
so you have to find out x3 and y3 through x and y on start(x1 and y1);
Something like that x3=y1.
Good luck and sorry if my English is not enough good for explaining it)