def cal_triangle_area(p1, p2, p3):
    """
    Calculate the area of triangle.
    S = |(x2 - x1)(y3 - y1) - (x3 - x1)(y2 - y1)| / 2
    :param p1: (x, y)
    :param p2: (x, y)
    :param p3: (x, y)
    :return: The area of triangle.
    """
    [x1, y1], [x2, y2], [x3, y3] = p1, p2, p3
    return abs((x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)) / 2