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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.ArrayList;
import java.util.Scanner;

public class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String[] convert = sc.nextLine().split(" ");
        int n = Integer.parseInt(convert[0]);
        int m = Integer.parseInt(convert[1]);
        
        if((n >= 3 && n <= 200000) && (m >= 1 && m <= 200000)){
            String[] str = new String[m];
            for(int i = 0; i < m; i++){
                str[i] = sc.nextLine();
            }

            ArrayList<String> from = new ArrayList<String>();
            ArrayList<String> to = new ArrayList<String>();
        
            for(int i = 0; i < str.length;i++){
                if(str[i].startsWith("1"))
                    from.add(str[i]);
                if(str[i].endsWith(Integer.toString(n)))
                    to.add(str[i]);
            }
            
            for(int i = 0; i < to.size(); i++){
                char t = to.get(i).charAt(0);
                for(int j = 0; j < from.size(); j++){
                    char f = from.get(j).charAt(2);
                    if(Character.toString(t).equals(Character.toString(f))){
                        System.out.println("POSSIBLE");
                        return;
                    }
                }
            }          
            System.out.println("IMPOSSIBLE");
        }
        
    } 
}