def compare(first_point, ref_point, compare_point):
    """
    I don't know exaclty what does this formula... anybody can explain ?
    """
    x2, y2 = first_point
    x1, y1 = ref_point
    x3, y3 = compare_point
    m = (y2 - y1 * 1.00001) / (x2 - x1 * 1.00001)
    return y3 - y1 - m * (x3 - x1)
