def make_goal(s):
    """Generate a goal board with an given size

    Args:
        s (int, s>=3): The size of goal board to generate, i.e. the width of the board. 
    
    Returns:
        goal (list): A 2D list representing tiles of the goal board, 
                    e.g., if the size given is 3, the goal is [[1,2,3],[4,5,6],[7,8,9]].
    """
    goal = []
    arr = [i for i in range(1, s*s)]
    arr.append(0)
    for i in range(s):
        tmp = []
        for j in range(s):
            tmp.append(arr[i*s + j])
        goal.append(tmp)
    return goal