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

/**
 * yukicoder no.13
 * @author scache
 *
 */
public class Main {
	public static void main(String[] args) {
		Main p = new Main();
	}

	public Main() {
		Scanner sc = new Scanner(System.in);
		int w = sc.nextInt();
		int h = sc.nextInt();
		int[][] field = new int[h+2][w+2];
		for(int i=1;i<=h;i++){
			for(int j=1;j<=w;j++){
				field[i][j] = sc.nextInt();
			}
		}
		solve(field);
	}

	public void solve(int[][] field) {
		int[][] visit = new int[field.length][field[0].length];
		for(int i=0;i<visit.length;i++)
			Arrays.fill(visit[i], -1);
		
		for(int i=1;i<=field.length-2;i++){
			for(int j=1;j<=field[i].length-2;j++){
				if(visit[i][j]>=0)
					continue;
				visit[i][j] = 0;
				if(bfs(field, visit, j, i)){
					System.out.println("possible");
					return;
				}
			}
		}
		System.out.println("impossible");
	}
	
	int[] dx = {-1, 0, 1, 0};
	int[] dy = {0, 1, 0, -1}; 

	private boolean bfs(int[][] field, int[][] visit, int cx, int cy){
		LinkedList<Point> q = new LinkedList<Point>();
		LinkedList<Point> from = new LinkedList<Point>();
		q.offer(new Point(cx, cy));
		from.offer(new Point(cx, cy));
		while(!q.isEmpty()){
			Point p = q.poll();
			Point f = from.poll();
			
			for(int i=0;i<4;i++){
				int nx = p.x + dx[i];
				int ny = p.y + dy[i];
				if(field[p.y][p.x]!=field[ny][nx])
					continue;
				
				if(visit[ny][nx]>0 && (nx!=f.x || ny!=f.y)){
					return true;
				}else if(visit[ny][nx]<0){
					q.offer(new Point(nx, ny));
					from.offer(new Point(p.x, p.y));
					visit[ny][nx] = visit[p.y][p.x]+1;
				}
			}
		}
		
		return false;
	}
	
	private class Point{
		public int x;
		public int y;
		public Point(int x, int y) {
			this.x = x;
			this.y = y;
		}
	}
}