import java.util.Scanner;

public class Main2 {
	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};
	
	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, -1);
			}
		}
		System.out.println("impossible");
	}
	
	static void solve(int y, int x, int d) {
		int number = M[y][x];
		F[y][x] = true;
		for(int i=0; i<4; i++) {
			if(d == 0 && i == 1) continue;
			if(d == 1 && i == 0) continue;
			if(d == 2 && i == 3) continue;
			if(d == 3 && i == 2) continue;
			int nx = x + dx[i];
			int ny = y + dy[i];
			if(nx < 0 || nx >= W || ny < 0 || ny >= H) continue;
			if(M[ny][nx] != number) continue;
			if(F[ny][nx]) {
				System.out.println("possible");
				System.exit(0);
			}
			if(F[ny][nx]) continue;
			solve(ny, nx, i);
		}
	}
}