import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Deque;
import java.util.HashMap;
import java.util.InputMismatchException;
import java.util.Map;
import java.util.PriorityQueue;
import java.math.BigInteger;
    
public class Main{

static final InputStream in=System.in;
static final PrintWriter out=new PrintWriter(System.out);
static final int INF=Integer.MAX_VALUE/2;
static final long LINF=Long.MAX_VALUE/2;
static int W,H,MAX_WH=100,gx,gy;
static int[][] map=new int[MAX_WH+1][MAX_WH+1];
static boolean[][] isSearched=new boolean[MAX_WH+1][MAX_WH+1];
static final int[] dx={-1,0,1,0};
static final int[] dy={0,-1,0,1};

public static void main(String[] args) throws IOException{
InputReader ir=new InputReader(in);
W=ir.nextInt();
H=ir.nextInt();
boolean flag=false;
for(int i=0;i<H;i++) map[i]=ir.toIntArray(W);
lo:
for(int i=0;i<H;i++){
for(int j=0;j<W;j++){
if(!isSearched[i][j]){
int cnt=0;
for(int k=0;k<4;k++){
int nx=i+dx[k];
int ny=j+dy[k];
if(nx>=0&&nx<H&&ny>=0&&ny<W&&map[i][j]==map[nx][ny]) cnt++;
}
if(cnt>=2&&canSurround(i,j,-1,-1)){
flag=true;
break lo;
}
}
}
}
out.println(flag?"possible":"impossible");
out.flush();
}

public static boolean canSurround(int cx,int cy,int px,int py){
isSearched[cx][cy]=true;
if(px<0&&py<0){
gx=cx; gy=cy;
}
else if(cx==gx&&cy==gy) return true;
for(int k=0;k<4;k++){
int nx=cx+dx[k];
int ny=cy+dy[k];
if(nx>=0&&nx<H&&ny>=0&&ny<W&&map[cx][cy]==map[nx][ny]&&(nx!=px||ny!=py)) if(canSurround(nx,ny,cx,cy)) return true;
}
return false;
}

static class InputReader {
private InputStream in;
private byte[] buffer=new byte[1024];
private int curbuf;
private int lenbuf;

public InputReader(InputStream in) {this.in=in;}
  
public int readByte() {
if(lenbuf==-1) throw new InputMismatchException();
if(curbuf>=lenbuf){
curbuf= 0;
try{
lenbuf=in.read(buffer);
}
catch (IOException e) {
throw new InputMismatchException();
}
if(lenbuf<=0)
return -1;
}
return buffer[curbuf++];
}

public boolean isSpaceChar(int c){return !(c>=33&&c<=126);}

private int skip() {int b; while((b = readByte())!=-1&&isSpaceChar(b)); return b;}

public String next() {
int b=skip();
StringBuilder sb=new StringBuilder();
while(!isSpaceChar(b)){
sb.appendCodePoint(b);
b=readByte();
}
return sb.toString();
}
 
public int nextInt() {
int c=readByte();
while (isSpaceChar(c)) c=readByte();
boolean minus=false;
if (c=='-') {
minus=true;
c=readByte();
}
int res=0;
do{
if(c<'0'||c>'9') throw new InputMismatchException();
res=res*10+c-'0';
c=readByte();
}while(!isSpaceChar(c));
return (minus)?-res:res;
}

public long nextLong() {
int c=readByte();
while (isSpaceChar(c)) c=readByte();
boolean minus=false;
if (c=='-') {
minus=true;
c=readByte();
}
long res = 0;
do{
if(c<'0'||c>'9') throw new InputMismatchException();
res=res*10+c-'0';
c=readByte();
}while(!isSpaceChar(c));
return (minus)?-res:res;
}

public int[] toIntArray(int n){
int[] a=new int[n];
for(int i=0;i<n;i++) a[i]=nextInt();
return a;
}
}
}