The graphs with eigenvalue -1

The eigenvalue of adjacent matrix of graph G is called its eigenvalue. On the basis of the second class of graph and third class of graph with eigenvalue -1,two new types of graphs with eigenvalue -1 are characterized. Let G be a graph with n(2) vertices,which contains a complete induced subgraph with m vertices. If some suitable conditions are satisfied for m and n ,then -1 is an eigenvalue of G. Let G be a graph with n(m) vertices. If the complement graph G C of G is isomorphic to a complete (m-1) partite graph and the union of some isolated points,then -1 is at least the eigenvalue of G with multiplicity n-m. And it is pointed out that there exist other graphs with eigenvalue -1.