Problems
Semi-complete graph
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 data
Contains the number of vertices n (1 ≤ n ≤ 100) and the number of edges m (1 ≤ m ≤ ** 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