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 Java (OpenJDK 11.0.6) (judge verdict: Time Limit Exceeded)

import java.util.*;
public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();

    int a[] =new int[m];
    int b[] =new int[m];
    int st = 0;
    int en = 0;
    boolean ju = false;
    
    for(int i = 0 ; i < m ; ++i){
      a[i] = sc.nextInt();
      b[i] = sc.nextInt();
    }
    /*for(int i = 0 ; i < m ; ++i){
      if(b[i]==n){
        ju = true;
        break;
      }
    }
    
    if(ju==false){
      System.out.println("IMPOSSIBLE");
      return;
    }
    
    ju = false;*/
    
    for(int i = 0 ; i < m ; ++i){
      if(b[i]==n){
        int mid = a[i];
      
      for(int j = 0 ; j < m ; ++j){
        if(a[j] == 1){
          if(mid == b[j]){
            ju = true;
            break;
          }      
        }
      }
      }
    }
    
    if(ju) System.out.println("POSSIBLE");
    else System.out.println("IMPOSSIBLE");

  }
}