CF216D Spider's Web

Paw the Spider is making a web. Web-making is a real art, Paw has been learning to do it his whole life. Let's consider the structure of the web. There are $ n $ main threads going from the center of the web. All main threads are located in one plane and divide it into $ n $ equal infinite sectors. The sectors are indexed from $ 1 $ to $ n $ in the clockwise direction. Sectors $ i $ and $ i+1 $ are adjacent for every $ i $ , $ 1

The first line contains integer $ n $ ( $ 3

Print a single integer — the number of unstable cells in Paw the Spider's web.