def calculate_centroid(cluster):

   """
   
   Calculates centroids of a single cluster

   cluster is a 2-dimensional list of data points, each data point being a list of two integers.

   Returns a centroid as a list of integers.

   """

   num_of_points = len(cluster)
   cent_x = sum(point[0] for point in cluster) / num_of_points
   cent_y = sum(point[1] for point in cluster) / num_of_points
   return [cent_x, cent_y]