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: Wrong Answer)

#include<cstdio>
#include<cstdlib>
#include<algorithm>
#include<cmath>
#include<string>
#include<cstring>
using namespace std;
int a[200005],b[200005];
int n,m;
int main(){
	while(~scanf("%d%d",&n,&m)){
		int flag = 0,cnt = 0;
		memset(a,0,sizeof(a));
		memset(b,0,sizeof(b));

		for(int i = 1;i<=m;i++){
			scanf("%d%d",&a[i],&b[i]);
		}
		for(int i = 0;i<=m;i++){
			if(a[i]==1){
				cnt = 1;
			}
			if(a[i]==1&&b[i]>flag){
				flag = b[i];
				cnt = 1;
			}
		}
		for(int i = 0;i<=m;i++){
			if(a[i]==flag){
				flag = b[i];
				cnt++;
				if(b[i]==n){
					printf("POSSIBLE\n");
					break;
				}
				else{
					printf("IMPOSSIBLE\n");
					break;
				}
			}
		}
		if(cnt!=2)
			printf("IMPOSSBILE\n");
	}
}