import java.util.*;
import java.util.stream.*;
import static java.lang.Math.*;
public class Main{
	public static int[] dx={-1,0,0,1};
	public static int[] dy={0,1,-1,0};
	public static void main(String... args){
		Scanner sc=new Scanner(System.in);
		int w=sc.nextInt();
		int h=sc.nextInt();
		int[][] m=new int[h][w];
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				m[i][j]=sc.nextInt();
			}
		}
		boolean[][] reached=new boolean[h][w];
		boolean ans=false;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(reached[i][j])continue;
				final int num=m[i][j];
				Deque<Cell> deq=new ArrayDeque<>();
				deq.add(new Cell(null,new Pair(i,j)));
				while(deq.size()>0){
					Cell cell=deq.poll();
					if(reached[cell.next.x][cell.next.y]){
						//put("(x,y)=(%d,%d)",cell.next.x,cell.next.y);
						put("possible");
						return;
					}
					reached[cell.next.x][cell.next.y]=true;
					for(int k=0;k<4;k++){
						int x=cell.next.x+dx[k];
						int y=cell.next.y+dy[k];
						if(x<0||x>=h||y<0||y>=w)continue;
						if(m[x][y]!=num)continue;
						Pair next=new Pair(x,y);
						if(next.equals(cell.prev))continue;
						deq.addLast(new Cell(cell.next,next));
					}
				}
			}
		}
		put("impossible");
	}
	public static class Cell{
		final Pair prev,next;
		Cell(Pair prev,Pair next){
			this.prev=prev;
			this.next=next;
		}
	}
	public enum Direction{
		UP,
		DOWN,
		LEFT,
		RIGHT;
		
	}
	public static class Pair{
		final int x,y;
		Pair(int x,int y){
			this.x=x;
			this.y=y;
		}
		@Override
		public String toString(){
			return String.format("Pair(%d,%d)",x,y);
		}
		@Override
		public int hashCode(){
			return x*1000+y;
		}
		@Override
		public boolean equals(Object obj){
			if(obj==null)return false;
			if(obj instanceof Pair){
				Pair pair=(Pair)obj;
				return x==pair.x&&y==pair.y;
			}
			return false;
		}	
	}
	public static void print(Object object){
        System.out.print(object);
    }
    public static void put(Object object) {
        System.out.println(object);
    }
    public static void put(){
        System.out.println();
    }
 
    public static void print(String format,Object... args){
        System.out.print(String.format(format,args));
    }
    public static void put(String format,Object... args) {
        System.out.println(String.format(format,args));
    }
}