Problems

# Semi-complete graph

# Semi-complete graph

A directed graph is called **semi-complete** if there is at least one edge between any pair of its distinct vertices.

For a graph given by a list of edges, check if it is semi-complete.

#### Input

Contains the number of vertices **n** (**1** ≤ **n** ≤ **100**) and the number of edges **m** (**1** ≤ **m** ≤ ** n** * (**n** - **1**)) in the graph. Then follows **m** pairs of numbers - the edges of the graph.

#### Output

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

Input example #1

5 10 1 2 1 3 1 5 2 3 2 5 3 2 4 1 4 3 4 5 5 3

Output example #1

NO