def heuristic(node, goal)->int:
    """
    Heuristic function that estimates the cost of the cheapest path from node to the goal.
    In our case it is simple manhatan distance.
    :param node:
    :param goal:
    :return: Manhatan distance of nodes
    """
    # expand nodes
    x1, y1 = node.get_pos()
    x2, y2 = goal.get_pos()
    # compute
    return abs(x1 - x2) + abs(y1 - y2)