using System;
using System.Diagnostics;
using System.Collections.Generic;
using System.Linq;
using Enu = System.Linq.Enumerable;

class Solution {
    int[] dx = new int[] { 1, 0 }, dy = new int[] { 0, 1 };
    int W, H;
    int[][] board;
    void calc() {
        rio(out W, out H);

        board = new int[H, W].ToJagArray();
        for(int i = 0; i < H; i++)
            board[i] = ria(W);

        var uf = new UnionFind(H * W);

        for(int i = 0; i < H; i++) {
            for(int j = 0; j < W; j++) {
                for(int k = 0; k < dx.Length; k++) {
                    int ni = i + dy[k], nj = j + dx[k];
                    if(ni < 0 || H <= ni || nj < 0 || W <= nj) continue;
                    if(board[i][j] != board[ni][nj]) continue;
                    if(uf.same(i * W + j, ni * W + nj)) {
                        Console.WriteLine("possible");
                        return;
                    }
                    else
                        uf.unite(i * W + j, ni * W + nj);
                }
            }
        }

        Console.WriteLine("impossible");
    }
    public class UnionFind {
        int vnum;
        int[] parent, rank;

        public UnionFind(int vnum) {
            this.vnum = vnum;
            parent = Enumerable.Range(0, vnum + 1).ToArray();
            rank = new int[vnum + 1];
        }

        public int find(int x) {
            if(parent[x] == x) return x;
            else return (parent[x] = find(parent[x]));
        }

        public void unite(int x, int y) {
            x = find(x);
            y = find(y);
            if(x == y) return;

            if(rank[x] < rank[y]) parent[x] = y;
            else {
                parent[y] = x;
                if(rank[x] == rank[y]) rank[x]++;
            }
        }

        public bool same(int x, int y) {
            return find(x) == find(y);
        }
    }

    static void Main(string[] args) {
        new Solution().calc();
    }

    #region
    static int ri() { return int.Parse(Console.ReadLine()); }
    static int[] ria(int n) {
        if(n <= 0) { Console.ReadLine(); return new int[0]; }
        else return Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
    }
    static void rio(out int p1) { p1 = ri(); }
    static void rio(out int p1, out int p2) { var r = ria(2); p1 = r[0]; p2 = r[1]; }
    static void rio(out int p1, out int p2, out int p3) { var r = ria(3); p1 = r[0]; p2 = r[1]; p3 = r[2]; }
    static void rio(out int p1, out int p2, out int p3, out int p4) { var r = ria(4); p1 = r[0]; p2 = r[1]; p3 = r[2]; p4 = r[3]; }
    static void rio(out int p1, out int p2, out int p3, out int p4, out int p5) { var r = ria(5); p1 = r[0]; p2 = r[1]; p3 = r[2]; p4 = r[3]; p5 = r[4]; }
    static long rl() { return long.Parse(Console.ReadLine()); }
    static long[] rla(int n) {
        if(n <= 0) { Console.ReadLine(); return new long[0]; }
        else return Console.ReadLine().Trim().Split(' ').Select(long.Parse).ToArray();
    }
    static void rlo(out long p1) { p1 = rl(); }
    static void rlo(out long p1, out long p2) { var r = rla(2); p1 = r[0]; p2 = r[1]; }
    static void rlo(out long p1, out long p2, out long p3) { var r = rla(3); p1 = r[0]; p2 = r[1]; p3 = r[2]; }
    static void rlo(out long p1, out long p2, out long p3, out long p4) { var r = rla(4); p1 = r[0]; p2 = r[1]; p3 = r[2]; p4 = r[3]; }
    static void rlo(out long p1, out long p2, out long p3, out long p4, out long p5) { var r = rla(5); p1 = r[0]; p2 = r[1]; p3 = r[2]; p4 = r[3]; p5 = r[4]; }
    static double rd() { return double.Parse(Console.ReadLine()); }
    static double[] rda(int n) {
        if(n <= 0) { Console.ReadLine(); return new double[0]; }
        else return Console.ReadLine().Trim().Split(' ').Select(double.Parse).ToArray();
    }
    static void rdo(out double p1) { p1 = rd(); }
    static void rdo(out double p1, out double p2) { var r = rda(2); p1 = r[0]; p2 = r[1]; }
    static void rdo(out double p1, out double p2, out double p3) { var r = rda(3); p1 = r[0]; p2 = r[1]; p3 = r[2]; }
    static void rdo(out double p1, out double p2, out double p3, out double p4) { var r = rda(4); p1 = r[0]; p2 = r[1]; p3 = r[2]; p4 = r[3]; }
    static void rdo(out double p1, out double p2, out double p3, out double p4, out double p5) { var r = rda(5); p1 = r[0]; p2 = r[1]; p3 = r[2]; p4 = r[3]; p5 = r[4]; }
    static void swap<T>(ref T x, ref T y) { T temp = x; x = y; y = temp; }
    static void wa1<T>(T[] a) { Debug.WriteLine(string.Join(" ", a)); }
    static void wa2<T>(T[][] a) {
        foreach(var row in a) {
            Debug.WriteLine(String.Join(" ", row));
        }
    }
    [DebuggerDisplay("{x} , {y}")]
    class point {
        public int x, y;
        public point(int x, int y) {
            this.x = x; this.y = y;
        }
    }
    #endregion
}

static class Extention {
    public static T[][] ToJagArray<T>(this T[,] a) {
        int n = a.GetLength(0), m = a.GetLength(1);
        var ret = new T[n][];
        for(int i = 0; i < n; i++) {
            ret[i] = new T[m];
            for(int j = 0; j < m; j++) {
                ret[i][j] = a[i, j];
            }
        }
        return ret;
    }
}