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 ≤ 10000) in the graph. Then m pairs of numbers are given - the graph edges.
Print YES if the graph is complete and NO otherwise.
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