A directed graph is called a tournament if there is exactly one edge between any pair of its different vertices. Graph is given by the list of edges. Check if it is a tournament.
The first line contains number of vertices n (1≤n≤100) and number of edges m (1≤m≤n⋅(n−1)/2) in the graph. The next m lines contain the pairs of numbers — the edges of the graph.
Print "YES" if graph is tournament and "NO" otherwise.