2003 AIME I Problem 12

In convex quadrilateral A B C D, \angle A \cong \angle C, A B=C D=180, and A D \neq B C. The perimeter of A B C D is 640 . Find \lfloor 1000 \cos A\rfloor. (The notation \lfloor x\rfloor means the greatest integer that is less than or equal to x.)