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: Wrong Answer)

use std::collections::HashSet;
fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    let v: Vec<usize> = s.trim().split_whitespace().map(|x| x.parse().unwrap()).collect();
    let mut x: Vec<usize> = Vec::new();
    let mut y: HashSet<usize> = HashSet::new();
    for _ in 0..v[1] {
        let mut s = String::new();
        std::io::stdin().read_line(&mut s).unwrap();
        let u: Vec<usize> = s.trim().split_whitespace().map(|x| x.parse().unwrap()).collect();
        if u[0] == 1 {x.push(u[1]);}
        if u[1] == v[0] {y.insert(u[0]);}
    }
    for i in &x {
        if match y.get(i) {Some(_) => true, None => false} {println!("POSSIBEL"); return}
    }
    println!("IMPOSSIBLE");
}