Undirected graph is called regular, if all its vertices have the same degree.
Graph is given by list of edges. Check, is it regular.
First line contains number n (1≤n≤100) of vertices and number m (m≤n⋅(n−1)/2) of edges in a graph. Then given m pairs of numbers — the edges of graph.
Print "YES" if graph is regular and "NO" otherwise.