using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using Enu = System.Linq.Enumerable;

class Program
{
    static readonly string Possible = "possible", Impossible = "impossible";
    static readonly int[] DR = { 0, 0, -1, 1 };
    static readonly int[] DC = { -1, 1, 0, 0 };
    int H, W;
    int[][] Grid;
    bool[,] Seen;

    void Solve()
    {
        W = reader.Int();
        H = reader.Int();
        Grid = reader.IntGrid(H);
        Seen = new bool[H, W];

        for (int r = 0; r < H; r++)
            for (int c = 0; c < W; c++)
            {
                Seen = new bool[H, W];
                if (DFS(r, c, -1, -1, r, c))
                {
                    Console.WriteLine(Possible);
                    return;
                }
            }
        Console.WriteLine(Impossible);
    }

    bool DFS(int r, int c, int prevR, int prevC, int startR, int startC)
    {
        Seen[r, c] = true;
        for (int d = 0; d < DR.Length; d++)
        {
            int nr = r + DR[d], nc = c + DC[d];
            if (nr == startR && nc == startC && (nr != prevR || nc != prevC)) return true;
            if (IsInside(nr, nc) && !Seen[nr, nc] && Grid[nr][nc] == Grid[r][c] &&
                DFS(nr, nc, r, c, startR, startC)) return true;
        }
        return false;
    }

    bool IsInside(int r, int c) { return r >= 0 && r < H && c >= 0 && c < W; }


    Reader reader = new Reader(Console.In);
    static void Main() { new Program().Solve(); }
}

class Reader
{
    private readonly TextReader reader;
    private readonly char[] separator = { ' ' };
    private readonly StringSplitOptions removeOp = StringSplitOptions.RemoveEmptyEntries;
    private string[] A = new string[0];
    private int i;

    public Reader(TextReader r) { reader = r; }
    public bool HasNext() { return Enqueue(); }
    public string String() { return Dequeue(); }
    public int Int() { return int.Parse(Dequeue()); }
    public long Long() { return long.Parse(Dequeue()); }
    public double Double() { return double.Parse(Dequeue()); }
    public int[] IntLine() { var s = Line(); return s == "" ? new int[0] : Array.ConvertAll(Split(s), int.Parse); }
    public int[] IntArray(int N) { return Enu.Range(0, N).Select(i => Int()).ToArray(); }
    public int[][] IntGrid(int H) { return Enu.Range(0, H).Select(i => IntLine()).ToArray(); }
    public string[] StringArray(int N) { return Enu.Range(0, N).Select(i => Line()).ToArray(); }
    public string Line() { return reader.ReadLine().Trim(); }
    private string[] Split(string s) { return s.Split(separator, removeOp); }
    private bool Enqueue()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return Enqueue();
        A = Split(line);
        i = 0;
        return true;
    }
    private string Dequeue() { Enqueue(); return A[i++]; }
}