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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

#define INF 1e9
#define MAXN 200005
#define MAXM 100005
#define ll long long
#define vi vector<int>
#define vll vector<long long>
#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define pii pair<int, int>

ll ans;
int N,M;
vi A, B;
bool visited[MAXN] = {}, flag=false;;

void solve(){
    rep(i, M){
        if(A[i] == 0) visited[B[i]] = true;
    }

    rep(i, M){
        if(B[i] == (N-1) && visited[A[i]]) {
            flag = true;
            break;
        }
    }

    if(flag) cout << "POSSIBLE" << endl;
    else cout << "IMPOSSIBLE" << endl;
}

int main(){
    cin >> N >> M;
    A.resize(M);
    B.resize(M);

    rep(i, M) {
        cin >> A[i] >> B[i];
        A[i]--, B[i]--;
    }
    solve();
}