Skylar is doing a part-time job recently. She needs to put up the posters on the billboards in SUSTech. All the billboards, one by one, form a long chain of different width and height. The posters are rectangular and they cannot overlap. Naturally, posters can have common points on the sides. Every poster should exactly cover the surface of the billboards and the whole area of billboards has to be covered. Skylar wants to minimize her workload, so she ask you to help her calculate the minimum number of required posters.