Problem:
Score : 300 points

Problem Statement

In Takahashi Kingdom, there is an archipelago of N islands, called Takahashi Islands.
For convenience, we will call them Island 1, Island 2, ..., Island N.

There are M kinds of regular boat services between these islands.
Each service connects two islands. The i-th service connects Island a_i and Island b_i.

Cat Snuke is on Island 1 now, and wants to go to Island N.
However, it turned out that there is no boat service from Island 1 to Island N, so he wants to know whether it is possible to go to Island N by using two boat services.

Help him.

Constraints

3 ≤ N ≤ 200 000

1 ≤ M ≤ 200 000

1 ≤ a_i < b_i ≤ N

(a_i, b_i) \neq (1, N)

If i \neq j, (a_i, b_i) \neq (a_j, b_j).

Input

Input is given from Standard Input in the following format:

N M
a_1 b_1
a_2 b_2
:
a_M b_M

Output

If it is possible to go to Island N by using two boat services, print POSSIBLE; otherwise, print IMPOSSIBLE.

Sample Input 1

3 2
1 2
2 3

Sample Output 1

POSSIBLE

Sample Input 2

4 3
1 2
2 3
3 4

Sample Output 2

IMPOSSIBLE

You have to use three boat services to get to Island 4.

Sample Input 3

100000 1
1 99999

Sample Output 3

IMPOSSIBLE

Sample Input 4

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

Sample Output 4

POSSIBLE

You can get to Island 5 by using two boat services: Island 1 -> Island 4 -> Island 5.

Solution in Perl (v5.18.2) (judge verdict: Wrong Answer)

use strict;
use warnings;
my $input = <STDIN>;
chomp $input;
my ($nn, $mm) = split / /, $input;
my %map;
 
my $amm = $mm;
while ($mm-- > 0) {
   $input = <STDIN>;
   chomp $input;
   my ($aa, $bb) = split / /, $input;
   $map{$aa} = $bb;
}
 
#for (keys %map) {
#    print "$_ : $map{$_}\n";
#}
 
my $cur = 1;
my $arr = 0;
my $cnt = 2;
#print "mm : $mm\n";
 
while ($cnt-- > 0) {
    
    $arr = $map{$cur};
    $arr == $nn ? do {print "Possible"; exit;} : do { $cur = $arr;}
}
print "Impossible";