#include <bits/stdc++.h>
#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it,X) for(auto it=(X).begin();it!=(X).end();it++)
#define numa(x,a) for(auto x: a)
#define ite iterator
#define mp make_pair
#define mt make_tuple
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define pb push_back
#define pf push_front
#define sec second
#define sz(x) ((int)(x).size())
#define ALL( c ) (c).begin(), (c).end()
#define gcd(a,b) __gcd(a,b)
#define mem(x,n) memset(x,n,sizeof(x))
#define endl "\n"
using namespace std;
template <int POS, class TUPLE> void deploy(std::ostream &os, const TUPLE &tuple){}
template <int POS, class TUPLE, class H, class ...Ts> void deploy(std::ostream &os, const TUPLE &t){ os << (POS == 0 ? "" : ", ") << get<POS>(t); deploy<POS + 1, TUPLE, Ts...>(os, t); }
template <class T> std::ostream& operator<<(std::ostream &os, std::vector<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T> std::ostream& operator<<(std::ostream &os, std::set<T> &v){ int remain = v.size(); os << "{"; for(auto e: v) os << e << (--remain == 0 ? "}" : ", "); return os; }
template <class T, class K> std::ostream& operator<<(std::ostream &os, std::map<T, K> &mp){ int remain = mp.size(); os << "{"; for(auto e: mp) os << "(" << e.first << " -> " << e.second << ")" << (--remain == 0 ? "}" : ", "); return os; }
#define DEBUG1(var0) { std::cerr << (#var0) << "=" << (var0) << endl; }
#define DEBUG2(var0, var1) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG1(var1); }
#define DEBUG3(var0, var1, var2) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG2(var1,var2); }
#define DEBUG4(var0, var1, var2, var3) { std::cerr << (#var0) << "=" << (var0) << ", ";DEBUG3(var1,var2,var3); }
using ll = long long;

int W,H;
int nums[110][110];
bool used[110][110];
int dx[] = {0,-1,1,0};
int dy[] = {-1,0,0,1};
int now;
bool search(int x, int y,int bx=-1,int by=-1){
  if(used[x][y]){
    return true;
  }
  used[x][y] = true;
  rep(i,4){
    int nx = x + dx[i];
    int ny = y + dy[i];
    if(bx == nx && by == ny){
      continue;
    }
    if(0 <= nx && nx< H && 0 <= ny && ny < W && nums[nx][ny] == now){
      if(search(nx,ny,x,y)){
        return true;
      }
    }
  }
  return false;
}
int main(){
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> W >> H;
  rep(i,H){
    rep(j,W){
      cin >> nums[i][j];
    }
  }
  bool OK = false;
  rep(i,H){
    rep(j,W){
      if(!used[i][j]){
        now = nums[i][j];
        if(search(i,j)){
          //DEBUG2(i,j);
          OK = true;
          goto result_out;
        }
      }
    }
  }
 result_out:
  if(OK){
    cout << "possible\n";
  }else{
    cout << "impossible\n";
  }
  return 0;
}