e-olymp
Competitions

Graphs Representation

Regular graph

Undirected graph is called regular, if all its vertices have the same degree.

Graph is given by list of edges. Check, is it regular.

Input

First line contains number n (1n100) of vertices and number m (mn (n - 1) / 2) of edges in a graph. Then given m pairs of numbers - the edges of graph.

Output

Print YES if graph is regular 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
Input example #2
3 2
1 2
2 3
Output example #2
NO