def get_area(pos):
    """
    Args
        pos: [B, N, 4]
            (x1, x2, y1, y2)

    Return
        area : [B, N]
    """
    # [B, N]
    height = pos[:, :, 3] - pos[:, :, 2]
    width = pos[:, :, 1] - pos[:, :, 0]
    area = height * width
    return area