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::cmp::max;
#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };

    ($next:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, [ $t:tt ]) => {
        {
            let len = read_value!($next, usize);
            (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        }
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, bytes) => {
        read_value!($next, String).into_bytes()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}
//usage
//input! {
//  h: i32;
//  v: [i32; n]
//  x: [[i32; n]; m]
//  f: chars <- Vec<char>
//}
use std::collections::BinaryHeap;

const INF: i64 = std::i64::MAX;

#[derive(Debug)]
struct Graph {
    node: usize,
    directed: bool,
    graph: Vec<Vec<(usize, i64)>>,
}

impl Graph {
    fn new(n: usize, flag: bool) -> Graph {
        Graph { node: n, directed: flag, graph: vec![vec![]; n] }
    }

    fn add_edge(&mut self, start_node: usize, end_node: usize, cost: i64) -> () {
        self.graph[start_node].push((end_node, cost));
        if !self.directed { self.graph[end_node].push((start_node, cost)); }
    }
    fn dijkstra(&self, start_node: usize) -> Vec<i64> {
        let mut dist = vec![INF; self.node];
        dist[start_node] = 0;

        let mut queue: BinaryHeap<(i64, usize)> = BinaryHeap::new();
        queue.push((0, start_node));

        while !queue.is_empty() {
            let (min_dist, idx) = queue.pop().unwrap();
            if dist[idx] < -min_dist {
                continue;
            }
            for &(to, weight) in &self.graph[idx] {
                let cost_cand = dist[idx] + weight;
                if dist[to] > cost_cand {
                    dist[to] = cost_cand;
                    queue.push((-cost_cand, to));
                }
            }
        }
        dist
    }
}
fn main() {
    input!{
        n: usize,
        m: usize,
        edge: [(usize, usize); m]
    }
    let mut g = Graph::new(n, false);
    for (a, b) in edge{
        g.add_edge(a - 1, b - 1, 1);
    }
    let mut dist = g.dijkstra(0);
    println!("{}", if dist[n - 1] != INF && dist[n - 1] <= 2{"POSSIBLE"} else{"IMPOSSIBLE"});
}