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D. Almost Acyclic Graph

time limit per test

1 secondmemory limit per test

256 megabytesinput

standard inputoutput

standard outputYou are given a directed graph consisting of *n* vertices and *m* edges (each edge is directed, so it can be traversed in only one direction). You are allowed to remove at most one edge from it.

Can you make this graph acyclic by removing at most one edge from it? A directed graph is called acyclic iff it doesn't contain any cycle (a non-empty path that starts and ends in the same vertex).

Input

The first line contains two integers *n* and *m* (2 ≤ *n* ≤ 500, 1 ≤ *m* ≤ *min*(*n*(*n* - 1), 100000)) — the number of vertices and the number of edges, respectively.

Then *m* lines follow. Each line contains two integers *u* and *v* denoting a directed edge going from vertex *u* to vertex *v* (1 ≤ *u*, *v* ≤ *n*, *u* ≠ *v*). Each ordered pair (*u*, *v*) is listed at most once (there is at most one directed edge from *u* to *v*).

Output

If it is possible to make this graph acyclic by removing at most one edge, print YES. Otherwise, print NO.

Examples

Input

3 4

1 2

2 3

3 2

3 1

Output

YES

Input

5 6

1 2

2 3

3 2

3 1

2 1

4 5

Output

NO

Note

In the first example you can remove edge , and the graph becomes acyclic.

In the second example you have to remove at least two edges (for example, and ) in order to make the graph acyclic.

Codeforces (c) Copyright 2010-2017 Mike Mirzayanov

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