def get_box_area(a1, a2):
    """
    Get the area of a box specified by two anchors
    
    Parameters
    ----------
    a1: list(2)
        Row/column of first anchor
    a2: list(2)
        Row/column of second anchor
    
    Returns
    -------
    Area of box determined by these two anchors
    """
    m = a2[0]-a1[0]+1
    n = a2[1]-a1[1]+1
    return m*n