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: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;


public class Main {

	/**
	 * @param args
	 * @throws IOException 
	 */
	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] tmpArray = br.readLine().split(" ");
		int numOfIsland = Integer.parseInt(tmpArray[0]);
		int numOfShip = Integer.parseInt(tmpArray[1]);
		
		ArrayList<Integer> shipFrom1 = new ArrayList<Integer>();
		ArrayList<Integer> shipToN = new ArrayList<Integer>();
		
		boolean result = false;
		
		for(int i = 0; i < numOfShip ; i++){
//			tmpArray = br.readLine().split(" ");
//			int a = Integer.parseInt(tmpArray[0]);
//			int b = Integer.parseInt(tmpArray[1]);
//			
			int a = 0;
			int b = 0;
			int num = 0;
			int tmp = br.read();
			int flag = 0;
			//a bの読み込み
			while(tmp != -1 && tmp != 10 && tmp != 13){
	            num = 0;
	            while(tmp >= '0' && tmp <= '9'){
	                num = num * 10 + (tmp - '0');
	                tmp = br.read();
	            }
	            if(tmp == -1){
	                break;
	            }
	            if(flag == 0){
	            	a = num;
	            	tmp = br.read();
	            }
	            else b = num;
//	            tmp = br.read();
	            flag++;
	        }
			
			//bの読み込み
//			num = 0;
//			tmp = br.read();
//			while(tmp != -1 && tmp != 32 && tmp != 10 && tmp != 13){
//	            num = 0;
//	            while(tmp >= '0' && tmp <= '9'){
//	                num = num * 10 + (tmp - '0');
//	                tmp = br.read();
//	            }
//	            if(tmp == -1){
//	                break;
//	            }
//	            b = num;
//	            tmp = br.read();
//	        }
//			
//			System.out.println(a + " " + b);
			
			if(a == 1){
				shipFrom1.add(b);
			}
			
			else if(b == numOfIsland){
				shipToN.add(a);
			}
		}
		
		//配列に変換
		Integer[] array1 = shipFrom1.toArray(new Integer[0]);
		Arrays.sort(array1);
		Integer[] array2 = shipToN.toArray(new Integer[0]);
		Arrays.sort(array2);

		if(array1.length <= array2.length){
			for(int i = 0; i < array1.length ; i++){
				if(Arrays.binarySearch(array2, array1[i]) >= 0){
					result = true;
					break;
				}
			}
		}
		else {
			for(int i = 0; i < array2.length ; i++){
				if(Arrays.binarySearch(array1, array2[i]) >= 0){
					result = true;
					break;
				}
			}
		}
		
//		
//		if(shipFrom1.size() <= shipToN.size()){
//			Iterator<Integer> it = shipFrom1.iterator();
//			for(int i = 0; i < shipFrom1.size() ; i++){
//				if(shipToN.contains(it.next())){
//					result = true;
//					break;
//				}
//			}
//		}
//		else {
//			Iterator<Integer> it = shipToN.iterator();
//			for(int i = 0; i < shipToN.size() ; i++){
//				if(shipFrom1.contains(it.next())){
//					result = true;
//					break;
//				}
//			}
//		}
//		
		if(result){
			System.out.println("POSSIBLE");
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

}