Taso has a long field, and he plans to plant different crops in the coming season. However, there are many crows in this area, and Taso is afraid that they will feed on crops. Therefore, he decided to put some scarecrows in different parts of the field. This field is divided into 1*n squares. Part of this land is barren, and they can't grow crops. Every scarecrow placed on a point has the smallest overlapping area with the scarecrows on his left and right sides.

According to the site description given, how many scarecrows are needed to cover the useful area in the whole site? Useful areas include squares where crops can grow.