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.util.Scanner;

public class Main {

    public static void main(String[] args) {
	Main m = new Main();
	m.answer();
    }

    private Scanner scan = new Scanner(System.in);
    private final int N;
    private final int M;
    private final boolean[] from1;
    private final boolean[] toN;

    public Main() {
	N = Integer.parseInt(scan.next());
	M = Integer.parseInt(scan.next());
	from1 = new boolean[N];
	toN = new boolean[N];
	for (int i = 0; i < M; i++) {
	    int a = Integer.parseInt(scan.next()) - 1;
	    int b = Integer.parseInt(scan.next()) - 1;
	    if(a == 0) from1[b] = true;
	    if(b == N-1) toN[a] = true;
	}

	scan.close();
    }

    public final void answer() {
	boolean ans = false;
	for (int i = 0; i < N; i++) {
	    if(from1[i] && toN[i]) {
		ans = true;
		break;
	    }
	}
	System.out.println(ans ? "POSSIBLE" : "IMPOSSIBLE");
    }
}