Undirected graph is called complete, if any pair of its different vertices is connected with at least one edge. For a given list of graph edges, check whether it is complete.
The first line contains the number of vertices n (1≤n≤100) and the number of edges m (1≤m≤104) in the graph. Then m pairs of numbers are given — the graph edges.
Print "YES" if the graph is complete and "NO" otherwise.