import java.util.*;
import java.lang.*;
import java.io.*;

public class Main {
    public static final String IMPOSSIBLE = "impossible";
    public static final String POSSIBLE = "possible";


    int W;
    int H;
    char[][] matrix;
    int[][] visited;

    //クラスを使わずにしてみたかったので
    int getPos(int y, int x) {
        return y * W + x;
    }

    void input() {
        Scanner scanner = new Scanner(System.in);
        //番兵を含む

        W = scanner.nextInt() + 2;
        H = scanner.nextInt() + 2;
        scanner.nextLine();
        matrix = new char[H][W];
        for (int i = 1; i < H - 1; i++) {
            for (int j = 1; j < W - 1; j++) {
                matrix[i][j] = (char) scanner.nextInt();

            }

        }
    }


    public Main() {
        input();
        visited = new int[H][W];
        for (int i = 0; i < H; i++) {
            Arrays.fill(visited[i], -1);
        }

        for (int i = 1; i < H - 1; i++) {
            for (int j = 1; j < W - 1; j++) {
                if (visited[i][j] != -1) {

                    continue;
                }
                if (bfs(i, j)) {
                    System.out.println(POSSIBLE);
                    return;
                }

            }
        }
        System.out.println(IMPOSSIBLE);
    }

    private boolean bfs(int startY, int startX) {
        LinkedList<Integer> queue = new LinkedList<Integer>();
        queue.add(getPos(startY, startX));
        char currentNum = matrix[startY][startX];
        visited[startY][startX] = 0;
        while (queue.size() > 0) {
            final Integer next = queue.pop();
            int x = next % (W);
            int y = next / (W);
            for (int i = -1; i <= 1; i++) {
                for (int j = -1; j <= 1; j++) {
                    int p2 = x + i;
                    int p1 = y + j;
//                    if (p1 == -1 || p2 == -1) {
//                        System.out.println("--"+x + ":" + y + "--" + next);
//                        System.exit(0);
//                    }


                    if (Math.abs(i) + Math.abs(j) == 1 && matrix[p1][p2] == currentNum) {
                        //次の場所が別の親になっているかどうかをチェックします。
                        if (visited[p1][p2] == 0 || visited[y][x] == getPos(p1, p2)) {
                            continue;
                        }

                        if (visited[p1][p2] != -1) {
                            //この時点で囲める
                            return true;
                        }
                        //次の場所の親が元の場所と設定します。
                        visited[p1][p2] = getPos(y, x);
                        int pos = getPos(p1, p2);

                        queue.add(pos);

                    }
                }
            }
        }
        return false;
    }

    public static void main(String[] args) {
        new Main();
    }
}