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

public class Main {
	
	public static final int UNVISITED = 0;
	public static final int VISITING = 1;
	public static final int VISITED = 2;
	
	public static final int[][] move_dirs = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
	public static boolean in_range(int x, int y, int W, int H){
		return 0 <= x && x < W && 0 <= y && y < H;
	}
	
	public static boolean dfs(int x, int y, int px, int py, int H, int W, int[][] board, int[][] visited){
		//System.out.println(y + " " + x + " " + board[y][x]);
		visited[y][x] = VISITING;
		
		for(final int[] move : move_dirs){
			final int nx = x + move[0];
			final int ny = y + move[1];
			
			if(!in_range(nx, ny, W, H)){ continue; }
			if(board[y][x] != board[ny][nx]){ continue; }
			if(ny == py && nx == px){ continue; }
			
			if(visited[ny][nx] == UNVISITED){
				if(dfs(nx, ny, x, y, H, W, board, visited)){
					visited[y][x] = VISITED;
					return true;
				}
			}else if(visited[ny][nx] == VISITING){
				visited[y][x] = VISITED;
				return true;
			}
		}
		
		visited[y][x] = VISITED;
		return false;
	}
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int W = sc.nextInt();
		final int H = sc.nextInt();
		
		int[][] board = new int[H][W];
		for(int i = 0; i < H; i++){
			for(int j = 0; j < W; j++){
				board[i][j] = sc.nextInt();
			}
		}
		
		int[][] visited = new int[H][W];
		
		for(int y = 0; y < H; y++){
			for(int x = 0; x < W; x++){
				if(visited[y][x] != UNVISITED){ continue; }
				
				if(dfs(x, y, -1, -1, H, W, board, visited)){
					System.out.println("possible");
					return;
				}
			}
		}
		
		System.out.println("impossible");
	}
}