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 C# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;

namespace MyProgram{
    class AtCoder{
        static void Main(){
            int[] input = Array.ConvertAll(Console.ReadLine().Split(' '),int.Parse);
            int N = input[0];
            int M = input[1];
            int[] a = new int[M];
            int[] b = new int[M];
            for(var i=0;i<M;i++){
                int[] input1 = Array.ConvertAll(Console.ReadLine().Split(' '),int.Parse);           
                a[i] = input1[0];
                b[i] = input1[1];
            }
            bool flag = false;
            var pos = new List<int>();
            for(var i=0;i<M;i++){
                if(a[i]==1){
                    pos.Add(b[i]);
                }
            }
            for(var i=0;i<M;i++){
                if(b[i]==N){
                    for(var j=0;j<pos.Count;j++){
                        if(pos[j]==a[i]){
                            flag = true;
                            goto END;
                        }
                    }
                }
            }
            END:
            Console.WriteLine(flag? "POSSIBLE":"IMPOSSIBLE");
        }
    } 
}