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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
int kuaipai(const void *aaaaaa, const void *bbbbbb)
{return *(int *)aaaaaa - *(int *)bbbbbb;}
int s[200005][5],a[200005],b[200005];

int main()
{
    int i,M,N,t,j=1,k=1,x=1,y=1;
    scanf("%d%d",&M,&N);
    for(i=1;i<=N;i++)
        scanf("%d%d",&s[i][1],&s[i][2]);
    for(i=1;i<=N;i++)
    {
        if(s[i][1]==1)
            a[j++]=s[i][2];
        if(s[i][2]==M)
            b[k++]=s[i][1];
    }
    qsort(a, j, sizeof(a[0]), kuaipai);
    qsort(b, k, sizeof(b[0]), kuaipai);
    for(t=1;t<=200000;t++)
    {
        if(a[x]>b[y])
            y++;
        if(a[x]<b[y])
            x++;
        if(a[x]==b[y])
        {printf("POSSIBLE\n");  return 0;}
    }
    printf("IMPOSSIBLE\n");
    return 0;
}