By Proposition 3.3, if there is no negative cycle reachable from s, d values equal opt values at all vertices after neg(G) 1 rounds of BFD. By Fact 3.1(2), at the round numbered at most neg(G) 2, there is no change of d values, and BFD stops. Since neg(G) V 3, this should happen not later than after round V 1.