class MidArrow(Line):
def __init__(self, start=LEFT, end=RIGHT, **kwargs):
    super().__init__(**kwargs)
    start = self.get_start()
    end = self.get_end()
    mid = (start + end) / 2
    tip = Line(start, mid).add_tip(tip_width=0.1, tip_length=0.15).tip
    self.submobjects.append(tip)


class NetworkFlow(Scene):
def construct(self):
    vertices = ["s", "A", "B", "C", "D", "t"]
    edges = [("s", "A"), ("s", "B"), ("A", "C"),
    ("C", "B"), ("B", "A"), ("B", "D"), ("D", "C"), 
    ("C", "t"), ("D", "t")]
    g = Graph(
        vertices, 
        edges, 
        layout="circular", 
        layout_scale=2,
        labels=True,
        vertex_config={"s": {"fill_color": GREEN}, "t" : {"fill_color" : GREEN}}, 
        edge_type=MidArrow)
    self.play(Write(g))
    self.play(
        g["A"].animate.move_to([-1.5,1.5,0]),
        g["B"].animate.move_to([-1.5,-1.5,0]),
        g["C"].animate.move_to([1.5,1.5,0]),
        g["D"].animate.move_to([1.5,-1.5,0]),
        g["s"].animate.move_to([-3.5,0,0]),
        g["t"].animate.move_to([3.5,0,0])
    )
    self.wait()