import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	static Scanner scan = new Scanner(System.in);
	static int W, H;
	static int[][] M;
	static boolean[][] F;
	static int[] dx = {0, 0, 1, -1};
	static int[] dy = {1, -1, 0, 0};
	static LinkedList<Node> q = new LinkedList<>();
	
	public static void main(String[] args) {
		W = scan.nextInt();
		H = scan.nextInt();
		M = new int[H][W];
		F = new boolean[H][W];
		for(int i=0; i<H; i++) {
			for(int j=0; j<W; j++) {
				M[i][j] = scan.nextInt();
			}
		}
		for(int i=0; i<H; i++) {
			for(int j=0; j<W; j++) {
				if(!F[i][j]) solve(i, j);
			}
		}
		System.out.println("impossible");
	}
	
	static void solve(int y, int x) {
		q.clear();
		q.add(new Node(x, y, -1));
		
		while(!q.isEmpty()) {
			Node now = q.poll();
			if(F[now.y][now.x]) {
				System.out.println("possible");
				System.exit(0);
			}
			int number = M[now.y][now.x];
			F[now.y][now.x] = true;
			
			for(int i=0; i<4; i++) {
				if(now.d == 0 && i == 1) continue;
				if(now.d == 1 && i == 0) continue;
				if(now.d == 2 && i == 3) continue;
				if(now.d == 3 && i == 2) continue;
				int nx = now.x + dx[i];
				int ny = now.y + dy[i];
				if(nx < 0 || nx >= W || ny < 0 || ny >= H) continue;
				if(F[ny][nx]) continue;
				if(M[ny][nx] != number) continue;
				q.add(new Node(nx, ny, i));
			}
		}
	}
	
	static class Node {
		int x, y, d;
		Node(int x, int y, int d) {
			this.x = x;
			this.y = y;
			this.d = d;
		}
	}
}