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 Java7 (OpenJDK 1.7.0) (judge verdict: Wrong Answer)

import java.util.*;
class Main{
	public static void main(String[] args){
		Scanner in = new Scanner(System.in);
		int N = Integer.parseInt(in.next());
		int M = Integer.parseInt(in.next());
		ArrayList<Integer> x = new ArrayList<Integer>();
		ArrayList<Integer> y = new ArrayList<Integer>();
		
		int tmpa,tmpb;
		for(int i = 0; i<M; i++){
			tmpa=Integer.parseInt(in.next());
			tmpb=Integer.parseInt(in.next());
			if(tmpa==1)x.add(tmpb);
			if(tmpb==1)x.add(tmpa);
			if(tmpa==N)y.add(tmpb);
			if(tmpb==N)y.add(tmpa);
		}
		Collections.sort(x);
		Collections.sort(y);
		String ans = "IMPOSSIBLE";
		for(int i = 0;i<x.size();i++){
			int j = 0;
			while(j<y.size()){
				if(x.get(i)==y.get(j)){
					ans = "POSSIBLE";
					break;
				}
				j++;
			}
			if(ans.equals("POSSIBLE"))break;
		}
		System.out.println(ans);
	}
}