Lionel and Lex

Problem description. Lex Luthor is a genius, albeit evil. Vertex Cover is an NP-hard problem. Lex Luthor's father, Lionel, is abusive. But even he knows that Lex cannot solve such a hard problem. He gives Lex an easier problem. Given the fact that in the graph G given to Lex, the Vertex-Cover has size strictly less than k or strictly greater than 2k, for a given integer k. Help poor Lex to tell t