def bbox_area(bbox):
    """Caluclulate bounding boxes' area
    
    Parameters
    ----------
    bbox: numpy.ndarray
        Numpy array containing bounding boxes of shape `N X 4` where N is the 
        number of bounding boxes and the bounding boxes are represented in the
        format `x1 y1 x2 y2`

    Returns
    -------
    
    numpy.ndarray
        Numpy array containing all bounding boxes's area of shape `N` where N is the 
        number of bounding boxes 
        format `area1 area2 ...` 
    """

    w = bbox[:, 2] - bbox[:, 0]
    h = bbox[:, 3] - bbox[:, 1]
    area = w * h

    return area