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: Accepted)

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


class Program {

	const long mod = 1000000007;
	//public static int[] max;

	static void Main(string[] args) {

		char[] cs = new char[] { ' ', ',' };
		//int val = int.Parse(Console.ReadLine());
		//int[] vals = parseAry(Console.ReadLine().Split(cs));
		//string str = Console.ReadLine();
		//string[] strs = Console.ReadLine().Split(cs);

		//int[] vals = parseAry(Console.ReadLine().Split(cs));
		//int n = vals[0];
		//string res="";

		//int y = vals[0];
		//int x = vals[1];

		int[] vals = parseAry(Console.ReadLine().Split(cs));
		int n = vals[0];
		int m = vals[1];


		//var to = new var[];
		//object[] to = new object[n + 1];
		var to = new List<List<int>>();

		for (int i = 0; i < n + 1; i++) {
			to.Add(new List<int>());
		}
		for (int i = 1; i < m+1; i++) {
			vals = parseAry(Console.ReadLine().Split(cs));
			to[vals[0]].Add(vals[1]);
		}

		int flg = 0;
			foreach (int a in to[1]) {
				foreach (int b in to[a]) {
					if (b == n) { flg = 1; break; }
				}
				
			}
			string re = "IMPOSSIBLE";
			if (flg == 1) { re="POSSIBLE";}

			Console.WriteLine(re);


		return;
	}



	//　String[]  ----->  int[]
	static int[] parseAry(string[] str) {
		int[] nums = new int[str.Length];
		for (int i = 0; i < str.Length; i++) { nums[i] = int.Parse(str[i]); }
		return nums;
	}
	//　String[]  ----->  long[]
	static long[] parseAryl(string[] str) {
		long[] nums = new long[str.Length];
		for (int i = 0; i < str.Length; i++) { nums[i] = int.Parse(str[i]); }
		return nums;
	}



	//文字列を1文字ずつString[]に入れる
	static string[] strAry(String str) {
		string[] re = new string[str.Length];
		for (int i = 0; i < str.Length; i++) { re[i] = str[i].ToString(); }
		return re;
	}

}