Alternating Groups I

There is a circle of red and blue tiles. You are given an array of integers colors. The color of tile i is represented by colors[i]:

Every 3 contiguous tiles in the circle with alternating colors (the middle tile has a different color from its left and right tiles) is called an alternating group.

Return the number of alternating groups.

Note that since colors represents a circle, the first and the last tiles are considered to be next to each other.