#include <bits/stdc++.h>

using namespace std;

int mat[111][111];
int w,h;

void yes(){
  cout<<"possible"<<endl;
  exit(0);
}

int sgn[111][111];
int seen;
int lstep=0;
void dfs(int y,int x,int step){
  if (y<0||x<0||h<=y||w<=x) return ;
  if (mat[y][x]!=seen) return ;
  if (sgn[y][x]!=0){
    if (abs(sgn[y][x]-step)<=1) return ;
    yes();
  }
  sgn[y][x]=++step;
  lstep=max(lstep,step);
  dfs(y-1,x,step);
  dfs(y+1,x,step);
  dfs(y,x-1,step);
  dfs(y,x+1,step);
  return ;
}

int main(){
  cin>>w>>h;
  for(int y=0;y<h;++y){
    for(int x=0;x<w;++x){
      scanf("%d",&(mat[y][x]));
    }}

  int t=0;
  for(int y=0;y<h;++y){
    for(int x=0;x<w;++x){
      if(sgn[y][x]==0){
	seen=mat[y][x];
	dfs(y,x,lstep+1);
      }
    }
  }
  cout<<"impossible"<<endl;
  
  return 0;
}