#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

vector< vector<int> > field;


void walk(int x, int y) {
	if (field[y][x] == 0) {
		return ;
	}

	int road = 0;
	int dir;
	for (int i = 0; i < 4; i++) {
		if (field[y+dy[i]][x+dx[i]] == field[y][x]) {
			dir = i;
			road++;
		}
	}
	if (road == 0) {
		field[y][x] = 0;
	} else if (road == 1) {
		field[y][x] = 0;
		walk(x+dx[dir], y+dy[dir]);
	}
}

int main() {
	int w, h;

	cin >> w >> h;
	field.assign(h + 2, vector<int>(w + 2, 0));
	for (int i = 1; i <= h; i++) {
		for (int j = 1; j <= w; j++) {
			cin >> field[i][j];
		}
	}

	for (int i = 1; i <= h; i++) {
		for (int j = 1; j <= w; j++) {
			walk(j, i);
		}
	}

	for (int i = 1; i <= h; i++) {
		for (int j = 1; j <= w; j++) {
			if (field[i][j] > 0) {
				cout << "possible" << endl;
				return 0;
			}
		}
	}
	cout << "impossible" << endl;
	return 0;
}