Graph tournament#5078
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 \le n \le 100) and number of edges m~(1 \le m \le n \cdot (n - 1)/2) in the graph. The next lines contain the pairs of numbers — the edges of the graph.
Print "YES" if graph is tournament and "NO" otherwise.
Example 1