Statement

Since he began his university studies in Milan, Edoardo started commuting by metro everyday. He usually goes to class by metro, but today he is so sleepy that he is not sure from which metro stop he started, nor which direction he took! It's worth to notice that trains in metro stations don't always open their doors from the same side: sometimes you need to get off from the left side, sometimes from the right side. There is a small map inside the train, which shows the position of the platform (left or right exit). With this information, Edoardo believes he can easily understand from which metro stop he started his commute. Edoardo paid attention during the last M stops, and for each stop he noted on which side the train opened its doors. Help Edoardo count how many possible starting metro stops are compatible with his observations...