**Problem**

Our brave Magicpig went to explore again! This time he was trapped in a
labyrinth! This labyrinth consists of n rooms numbered from 1 to n. There are
some tunnels connecting these rooms. But note that all the tunnels are one-way!
That means if there is a tunnel connecting room i and room j, you can only move
from room i to room j through this tunnel and it is impractical conversely. Magicpig was in room 1. On the wall of the room there were some ancient Egyptian
characters: "The only exit is in this room. If you leave this room, the door of
the exit will open. Once this door is opened, it won't close again. " Can
Magicpig escape from this labyrinth?

**Input**

The input contains one or more data sets. At first line of each input data set
there is two integers n and m, where n is the number of rooms and m is the
number of the tunnels. The next m lines contain integer pairs i and j, which
indicate that there is a tunnel between room i and room j and you can only move
from room i to room j through this tunnel.

2<=n<=100,1<=m<=n*(n-1),1<=i,j<=n

Zero at line for number of rooms terminates the input for your program.

**Output**

For each data set, if Magicpig can escape, print a string "YES", else print
"NO".

**Sample Input**

3 3

1 2

2 3

3 1

0

**Sample Output**

YES

Note: In the sample input, Magicpig first moves to room 2, then the door will
open. Now Magicpig can move to room 3 and then room 1 and escapes successfully!.

**Author: Mathematica**

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