import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.management.ManagementFactory;
import java.util.Arrays;
import java.util.function.IntFunction;

class Main{
  boolean isDebug = ManagementFactory.getRuntimeMXBean().getInputArguments().toString()
      .contains("-agentlib:jdwp");
  final MyReader in = new MyReader(System.in);
  final MyWriter out = new MyWriter(System.out);

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

  private void exe(){
    input();
    preCalc();
    solve();
    out.flush();
  }

  int W = in.it();
  int H = in.it();
  int[][] M = in.it(H,W);

  private void input(){}

  private void preCalc(){}

  void solve(){
    UnionFind uf = new UnionFind(H *W);
    for (int i = 0;i < H;i++)
      for (int j = 0;j < W;j++) {
        int p = i *W +j;
        if (i +1 < H && M[i][j] == M[i +1][j]) {
          int n = p +W;
          if (uf.same(p,n)) {
            out.println(true);
            return;
          }
          uf.merge(p,n);
        }

        if (j +1 < W && M[i][j] == M[i][j +1]) {
          int n = p +1;
          if (uf.same(p,n)) {
            out.println(true);
            return;
          }
          uf.merge(p,n);
        }

      }

    out.println(false);

  }

  static class UnionFind{
    final int[] par;
    final int[] size;

    public UnionFind(final int n){
      par = new int[n];
      Arrays.setAll(par,i -> i);
      size = new int[n];
      Arrays.fill(size,1);
    }

    int root(final int x){ return x == par[x] ? x : (par[x] = root(par[x])); }

    boolean same(final int u,final int v){ return root(u) == root(v); }

    void merge(final int x,final int y){
      int px = root(x);
      int py = root(y);
      if (px == py)
        return;
      if (size[px] < size[py]) {
        final int t = px;
        px = py;
        py = t;
      }

      par[py] = px;
      size[px] += size[py];
    }

    int size(final int x){ return size[root(x)]; }

  }

  /* 定数 */
  //  final static int mod = (int) 1e9 +7;
  final static int mod = 998244353;
  final static String yes = "possible";
  final static String no = "impossible";

  /* 入力 */
  static class MyReader{
    byte[] buf = new byte[1 <<16];
    int head = 0;
    int tail = 0;
    InputStream in;

    public MyReader(final InputStream in){ this.in = in; }

    byte read(){
      if (head == tail) {
        try {
          tail = in.read(buf);
        } catch (IOException e) {
          e.printStackTrace();
        }
        head = 0;
      }
      return buf[head++];
    }

    boolean isPrintable(final byte c){ return 32 < c && c < 127; }

    boolean isNum(final byte c){ return 47 < c && c < 58; }

    byte nextPrintable(){
      byte ret = read();
      return isPrintable(ret) ? ret : nextPrintable();
    }

    int it(){ return (int) lg(); }

    int[] it(final int N){
      int[] a = new int[N];
      Arrays.setAll(a,i -> it());
      return a;
    }

    int[][] it(final int H,final int W){ return arr(new int[H][],i -> it(W)); }

    int idx(){ return it() -1; }

    int[] idx(final int N){
      int[] a = new int[N];
      Arrays.setAll(a,i -> idx());
      return a;
    }

    int[][] idx(final int H,final int W){ return arr(new int[H][],i -> idx(W)); }

    long lg(){
      byte i = nextPrintable();
      boolean negative = i == 45;
      long n = negative ? 0 : i -'0';
      while (isPrintable(i = read()))
        n = 10 *n +i -'0';
      return negative ? -n : n;
    }

    long[] lg(final int N){
      long[] a = new long[N];
      Arrays.setAll(a,i -> lg());
      return a;
    }

    long[][] lg(final int H,final int W){ return arr(new long[H][],i -> lg(W)); }

    char[] ch(){ return str().toCharArray(); }

    char[][] ch(final int H){ return arr(new char[H][],i -> ch()); }

    String line(){
      StringBuilder sb = new StringBuilder();

      byte c;
      while (isPrintable(c = read()) || c == ' ')
        sb.append((char) c);
      return sb.toString();
    }

    String str(){
      StringBuilder sb = new StringBuilder();
      sb.append((char) nextPrintable());
      byte c;
      while (isPrintable(c = read()))
        sb.append((char) c);
      return sb.toString();
    }

    String[] str(final int N){ return arr(new String[N],i -> str()); }

    <T> T[] arr(final T[] arr,final IntFunction<T> f){
      Arrays.setAll(arr,f);
      return arr;
    }
  }

  /* 出力 */
  static class MyWriter{
    OutputStream out;

    byte[] buf = new byte[1 <<16];
    byte[] ibuf = new byte[20];

    int tail = 0;

    public MyWriter(final OutputStream out){ this.out = out; }

    void flush(){
      try {
        out.write(buf,0,tail);
        tail = 0;
      } catch (IOException e) {
        e.printStackTrace();
      }
    }

    void sp(){ write((byte) ' '); }

    void ln(){ write((byte) '\n'); }

    void write(final byte b){
      buf[tail++] = b;
      if (tail == buf.length)
        flush();
    }

    void write(final byte[] b,final int off,final int len){
      for (int i = off;i < off +len;i++)
        write(b[i]);
    }

    void write(long n){
      if (n < 0) {
        n = -n;
        write((byte) '-');
      }

      int i = ibuf.length;
      do {
        ibuf[--i] = (byte) (n %10 +'0');
        n /= 10;
      } while (n > 0);

      write(ibuf,i,ibuf.length -i);
    }

    void println(final boolean b){ println(b ? yes : no); }

    void println(final long n){
      write(n);
      ln();
    }

    public void println(final double d){ println(String.valueOf(d)); }

    void println(final String s){
      for (byte b:s.getBytes())
        write(b);
      ln();
    }

    public void println(final char[] s){
      for (char b:s)
        write((byte) b);
      ln();
    }

    void println(final int[] a){
      for (int i = 0;i < a.length;i++) {
        if (0 < i)
          sp();
        write(a[i]);
      }
      ln();
    }

    void println(final long[] a){
      for (int i = 0;i < a.length;i++) {
        if (0 < i)
          sp();
        write(a[i]);
      }
      ln();
    }

  }
}