Problems

# Complete graph

# Complete graph

Undirected graph is called *complete*, if any pair of its different vertices is connected with at least one edge. For a given list of graph edges, check whether it is complete.

#### Input

The first line contains the number of vertices **n** (**1** ≤ **n** ≤ **100**) and the number of edges **m** (**1** ≤ **m** ≤ **10000**) in the graph. Then **m** pairs of numbers are given - the graph edges.

#### Output

Print **YES** if the graph is complete and **NO** otherwise.

Input example #1

3 3 1 2 1 3 2 3

Output example #1

YES

Input example #2

5 18 1 2 1 3 1 3 1 4 1 4 1 4 1 5 1 5 2 3 2 4 2 4 2 5 3 4 3 4 3 4 3 5 3 5 4 5

Output example #2

YES