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 data
The first line contains the number of vertices n~(1 \le n \le 100) and the number of edges m~(1 \le m \le 10^4) in the graph. Then m pairs of numbers are given — the graph edges.
Output data
Print "YES" if the graph is complete and "NO" otherwise.
Examples
Input example #1
3 3 1 2 1 3 2 3
Output example #1
YES