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 Java (OpenJDK 1.8.0) (judge verdict: Accepted)


import java.io.*;
import java.util.*;
//import java.util.Map.Entry;
//import java.util.stream.*;
//import java.math.*;

public class Main {

    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
        Scanner sc = new Scanner(System.in);
        long MOD = (long) (Math.pow(10, 9) + 7);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int array[] = new int[n + 1];
        for (int i = 0; i < m; i++) {
            int ai = sc.nextInt();
            int bi = sc.nextInt();
            if (ai == 1) {
                array[bi]++;
            }
            if (bi == n) {
                array[ai]++;
            }
        }
        for (int num : array) {
            if (num == 2) {
                System.out.println("POSSIBLE");
                return;
            }
        }
        System.out.println("IMPOSSIBLE");
        bw.flush();
    }
//==============================================//
//==============================================//
//==============================================//
//==============================================//
//==============================================//
//==============================================//
//==============================================//
//==============================================//

    static class Pair implements Comparable<Pair> {

        int a, b;

        public Pair(int x, int y) {
            a = x;
            b = y;
        }

        @Override
        public int compareTo(Pair p) {
            return a - p.a;
        }
    }

    static void forl_0(int n, int[] array, Scanner sc) {
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
    }

    static void forl_1(int n, int[] array, Scanner sc) {
        for (int i = 1; i <= n; i++) {
            array[i] = sc.nextInt();
        }
    }

    static void two_d(int n, int m, int[][] array, Scanner sc) {
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                array[i][j] = sc.nextInt();
            }
        }
    }

    static boolean isPrime(long n) {
        if (n < 2) {
            return false;
        } else if (n == 2 || n == 3) {
            return true;
        } else if (n % 2 == 0 || n % 3 == 0) {
            return false;
        }
        long sqrtN = (long) Math.sqrt(n) + 1;
        for (long i = 6L; i <= sqrtN; i += 6) {
            if (n % (i - 1) == 0 || n % (i + 1) == 0) {
                return false;
            }
        }
        return true;
    }

    public static long gcd(long a, long b) {
        return (b == 0) ? a : gcd(b, a % b);
    }

    public static long lcm(long a, long b) {
        return a / gcd(a, b) * b;
    }
}