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>

#define N_MAX 200000

int n, m, a, b, i, is;
int is_from_one[N_MAX + 1];
int is_to_n[N_MAX + 1];

int main(void) {
    scanf("%d%d", &n, &m);
    
    for (i = 1; i <= m; ++i) {
        scanf("%d%d", &a, &b);
        if (a == 1) {
            is_from_one[b] = 1;
        } else if (b == n) {
            is_to_n[a] = 1;
        }
    }
    
    for (i = 1; i <= n; ++i) {
        //printf("%d from1, to n = %d, %d\n", i, is_from_one[i], is_to_n[i]);
        
        if (is_from_one[i] && is_to_n[i]) {
            is = 1;
        }
    }
    
    if (is) {
        puts("POSSIBLE");
    } else {
        puts("IMPOSSIBLE");
    }
    
    return 0;
}