FI-MO 2015 Problem 4

Let $n$ be a positive integer. Every square in a $n \times n$ -square grid is either white or black. How many such colourings exist, if every $2 \times 2$ -square consists of exactly two white and two black squares? The squares in the grid are identified as e.g. in a chessboard, so in general colourings obtained from each other by rotation are different.