e-olymp
Problems

Complete graph

Complete graph

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.

Input

The first line contains the number of vertices n (1n100) and the number of edges m (1m10000) in the graph. Then m pairs of numbers are given - the graph edges.

Output

Print YES if the graph is complete and NO otherwise.

Time limit 1 second
Memory limit 128 MiB
Input example #1
3 3
1 2
1 3
2 3
Output example #1
YES