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Problems

Semi-complete graph

Semi-complete graph

Time limit 1 second
Memory limit 128 MiB

A directed graph is called semi-complete if there is at least one edge between any pair of its distinct vertices.

For a graph given by a list of edges, check if it is semi-complete.

Input data

Contains the number of vertices n (1n100) and the number of edges m (1m ≤ ** n** * (n - 1)) in the graph. Then follows m pairs of numbers - the edges of the graph.

Output data

Print YES if the graph is semi-complete and NO otherwise.

Examples

Input example #1
5 10
1 2
1 3
1 5
2 3
2 5
3 2
4 1
4 3
4 5
5 3
Output example #1
NO