//yukicoder@cpp17

#include <iostream>
#include <iomanip>

#include <algorithm>

#include <cmath>
#include <cctype>
#include <climits>
#include <cassert>

#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>

#include <random>

#include <bitset>

#include <complex>

#include <utility>

#include <numeric>

#include <functional>


using namespace std;
using ll = long long;
using P = pair<ll,ll>;

const ll MOD = 998244353;
const ll MODx = 1000000007;
const int INF = (1<<30)-1;
const ll LINF = (1LL<<62LL)-1;
const double EPS = (1e-10);

P ar4[4] = {{0,1},{0,-1},{1,0},{-1,0}};
P ar8[8] = {{-1,-1},{-1,0},{-1,1},{0,-1},{0,1},{1,-1},{1,0},{1,1}};




template <typename T> vector<T> make_vector(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_vector(size_t a, Ts... ts) { return vector<decltype(make_vector(ts...))>(a, make_vector(ts...)); }
 
/*
確認ポイント
cout << fixed << setprecision(n) << 小数計算//n桁の小数表記になる

計算量は変わらないが楽できるシリーズ
min(max)_element(iter,iter)で一番小さい(大きい)値のポインタが帰ってくる
count(iter,iter,int)でintがiterからiterの間にいくつあったかを取得できる
*/

/* comment outed because can cause bugs
__attribute__((constructor))
void initial() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}
*/

int A[110][110];
bool V[110][110];
int w,h;
set<pair<int,int>> Z;
bool dfs(int cy, int cx, int v, int py = -1, int px = -1){
  if(cy+1 < h && !(py == cy+1 && px == cx) && A[cy+1][cx] == v){
    if(Z.count({cy+1, cx}))return true;
    if(!V[cy+1][cx]){
      V[cy+1][cx] = 1;
      Z.insert({cy+1, cx});
      if(dfs(cy+1, cx, v, cy, cx))return true;
      Z.erase({cy+1, cx});
    }
  }
  if(cx+1 < w && !(py == cy && px == cx+1) && A[cy][cx+1] == v){
    if(Z.count({cy, cx+1}))return true;
    if(!V[cy][cx+1]){
      V[cy][cx+1] = 1;
      Z.insert({cy, cx+1});
      if(dfs(cy, cx+1, v, cy, cx))return true;
      Z.erase({cy, cx+1});
    }
  }
  if(0 <= cy-1 && !(py == cy-1 && px == cx) && A[cy-1][cx] == v){
    if(Z.count({cy-1, cx}))return true;
    if(!V[cy-1][cx]){
      V[cy-1][cx] = 1;
      Z.insert({cy-1, cx});
      if(dfs(cy-1, cx, v, cy, cx))return true;
      Z.erase({cy-1, cx});
    }
  }
  if(0 <= cx-1 && !(py == cy && px == cx-1) && A[cy][cx-1] == v){
    if(Z.count({cy, cx-1}))return true;
    if(!V[cy][cx-1]){
      V[cy][cx-1] = 1;
      Z.insert({cy, cx-1});
      if(dfs(cy, cx-1, v, cy, cx))return true;
      Z.erase({cy, cx-1});
    }
  }

  return false;

}

int main(){
  cin>>w>>h;
  for(int i = 0; h > i; i++){
    for(int j = 0; w > j; j++){
      cin>>A[i][j];
    }
  }
  for(int i = 0; h > i; i++){
    for(int j = 0; w > j; j++){
      if(V[i][j] == 0){
        V[i][j] = 1;
        Z.insert({i,j});
        if(dfs(i,j, A[i][j])){
          cout << "possible" << endl;
          return 0;
        }
        Z.erase({i,j});
      }
    }
  }

  cout << "impossible" << endl;
  return 0;
}