def cross(o, a, b):
    """
    Calculates cross between two vectors.

    :param o, a: vector
    :param o, b: vector
    :return: cross product
    """
    ox, oy = o
    ax, ay = a
    bx, by = b

    return (ax - ox) * (by - oy) - (ay - oy) * (bx - ox)