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: Wrong Answer)

using System;
using System.Linq;
using System.Collections.Generic;

public class Test
{
	public static void Main()
	{
		string[] buffer = Console.ReadLine().Split(' ');
        int n = int.Parse(buffer[0]);
        int m = int.Parse(buffer[1]);
        string s1 = "";
        string s2 = "";
        if(m == 1)
        {
            Console.WriteLine("IMPOSSIBLE");
        }
        else
        { 
  
            string line;
            for(int i=0;i<m;i++)
            {
                line = Console.ReadLine();
                if(line.StartsWith("1")){
                    s1 += line;
                }
                else if(line.EndsWith(buffer[0])){
                    s2 += line;
                }
            }
        }
        bool b = false;
        string[] str1 = s1.Split(' ');
        string[] str2 = s2.Split(' ');
        List<string> list1 = new List<string>(str1);
        List<string> list2 = new List<string>(str2);
        List<string> newList = list1.FindAll(list2.Contains);
        int cnt = newList.Count;
        if(cnt > 0){
            b = true;
        }

        if(b == true){
            Console.WriteLine("POSSIBLE");
        } else {
            Console.WriteLine("IMPOSSIBLE");
        }
    }


}