We study undirected graphs without loops or multiple edges. Given a graph G;
V (G), E(G), v(G) and e(G) stands for the set of vertices, the set of edges, the
order (number of vertices) and the size (number of edges) of G. Kn, and Cn
stand for the complete graph and the cycle of order n. For two graphs G and
H we denote by G + H the graph obtained from the disjoint union G ∪ H by
adding all edges between G and H.