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 OCaml (4.02.3) (judge verdict: Accepted)

let n, m = Scanf.scanf "%d %d\n" (fun n m -> (n, m))

let hash = Hashtbl.create m

let rec init_hash m =
  if m = 0 then ()
  else
    let a, b = Scanf.scanf "%d %d\n" (fun a b -> (a, b)) in
    let ls = b :: (try Hashtbl.find hash a with Not_found -> []) in
    Hashtbl.replace hash a ls ;
    init_hash (m - 1)


let rec iter_search = function
  | [] -> "IMPOSSIBLE"
  | from_first :: rest ->
    let to_n = (try Hashtbl.find hash from_first with Not_found -> []) in 
    if List.exists (( = ) n)  to_n then "POSSIBLE"
    else iter_search rest


let () =
  let () = init_hash m in
  Hashtbl.find hash 1 |> iter_search |> Printf.printf "%s\n"