Problems

# Semi-complete graph

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

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

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

Time limit 1 second
Memory limit 128 MiB
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