def convert_coord(x_center, y_center, radius):
    """
    Convert coordinates from central point to top left point
    :param x_center: x coordinate of the center
    :param y_center: y coordinate of the center
    :param radius: the radius of the ball
    :return: coordinates of top left point of the surface
    """
    x = x_center - radius
    y = y_center - radius

    return x, y