Find Min Vertices to Remove(keteki)

Mohdevali is walking on a road which is acyclic if seen from graph theory perspective, suddenly he had a great observation. He found that if he travels at the speed of sound and shouts the number of intersections in the crossroads he can hear his own shouts from behind (cause he is travelling at the speed of sound). He doesn't like to hear big numbers but is ok traveling at big speeds. So he start