SPOJ Problem Set (classical)
1436. Is it a tree
Problem code: PT07Y

You are given an unweighted, undirected graph.
Write a program to check if it's a tree topology.
Input
The first line of the input file contains two integers N and M  number of nodes and number of edges in the graph
(0 < N <= 10000, 0 <= M <= 20000). Next M lines contain M edges of that graph  Each line contains a pair (u, v) means
there is an edge between node u and node v (1 <= u,v <= N).
Output
Print YES if the given graph is a tree, otherwise print NO.
Example
Input:
3 2
1 2
2 3
Output:
YES
