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 Rust (1.15.1) (judge verdict: Accepted)

use std::io;
use std::io::{Read, BufReader};

macro_rules! read {
    ( $( $t:ty ),* ) => {
        {
            let mut buf = String::new();
            io::stdin().read_line(&mut buf);
            let mut sp = buf.trim().split(" ");
            ($(
                {
                    sp.next().unwrap().parse::<$t>().unwrap()
                },
            )*)
        }
    }
}
macro_rules! read_vec {
    ($t:ty) => {
        {
            let mut buf = String::new();
            io::stdin().read_line(&mut buf);
            let mut sp = buf.trim().split(" ");
            let mut ret = vec![];
            for s in sp {
                ret.push(s.parse::<$t>().unwrap());
            }
            ret
        }
    }
}

fn main() {
    let (n, m) = read!(usize, usize);
    let mut ca = vec![false; n + 1];
    let mut cb = vec![false; n + 1];
    for _ in 0..m {
        let (a, b) = read!(usize, usize);
        if a == 1 {
            ca[b] = true;
        }
        if a == n {
            cb[b] = true;
        }
        if b == 1 {
            ca[a] = true;
        }
        if b == n {
            cb[a] = true;
        }
    }
    for i in 1..(n+1) {
        if ca[i] && cb[i] {
            println!("POSSIBLE");
            return;
        }
    }
    println!("IMPOSSIBLE");
}