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 (1.42.0) (judge verdict: Accepted)

use std::io::*;
use std::str::FromStr;
use std::cmp;
use std::collections::HashMap;
use std::collections::HashSet;
use std::char;
use std::cmp::{min, max};
use itertools::Itertools;
use std::ops::{Add, AddAssign};
use num_integer::{Integer, gcd, lcm};
use std::collections::VecDeque;
use smallvec::alloc::collections::LinkedList;

fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

fn read_tuple2<T: FromStr, S: FromStr>() -> (T, S) {
    let a = read();
    let b = read();
    (a, b)
}

fn read_tuple3<T: FromStr, S: FromStr, R: FromStr>() -> (T, S, R) {
    let a = read();
    let b = read();
    let c = read();
    (a, b, c)
}

fn lower_bound<T: Copy, F>(v: &Vec<T>, f: F) -> Option<usize> where
    F: Fn(T) -> bool
{
    let mut left = 0;
    let mut right = v.len() - 1;
    if !f(v[left]) {
        return None;
    }
    if f(v[right]) {
        return Some(right);
    }
    loop {
        if right - left == 1 {
            return Some(left);
        }
        let mid = (left + right) / 2;
        if f(v[mid]) {
            left = mid;
        } else {
            right = mid;
        }
    }
}

fn main() {
    let (n, m):(u32, u32) = read_tuple2();
    let routes:Vec<(u32, u32)> = (0..m).map(|_|read_tuple2()).collect();
    let mut route_maps = HashMap::new();
    for (from, to) in routes {
        let mut ent =
            route_maps.entry(from).or_insert(LinkedList::new());
        (*ent).push_back(to);
        let mut ent =
            route_maps.entry(to).or_insert(LinkedList::new());
        (*ent).push_back(from);
    }
    let mut snd = LinkedList::new();
    for i in route_maps.entry(1).or_insert(LinkedList::new()) {
        if *i == n {
            println!("POSSIBLE");
            return;
        }
        snd.push_back(*i);
    }
    for i in snd {
        for j in route_maps.entry(i).or_insert(LinkedList::new()) {
            if *j == n {
                println!("POSSIBLE");
                return;
            }
        }
    }
    println!("IMPOSSIBLE");
}