Problems

# Transitivity of a graph

# Transitivity of a graph

Undirected graph without loops and multiple edges is called *transitive*, if from conditions that vertices **u** and **v** are connected with an edge, vertices **v** and **w** are connected with an edge and all three vertices **u**, **v** and **w** are different, implies that vertices **u** and **w** are connected with an edge.

Verify that a given undirected graph is transitive.

#### Input

The first line contains the number of vertices **n** and edges **m** of the graph (**3** ≤ **n** ≤ **100**, **0** ≤ **m** ≤ (**n** · (**n** - **1**)) / **2**). Then **m** lines are given - the list of edges.

#### Output

Print **YES** or **NO** - the answer to the question about transitivity of a graph.

Input example #1

3 3 1 2 2 3 1 3

Output example #1

YES

Input example #2

3 2 1 2 1 3

Output example #2

NO