// includes
#include <cstdio>
#include <cstdint>
#include <iostream>
#include <iomanip>
#include <string>
#include <queue>
#include <stack>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <utility>
#include <functional>
#include <cmath>
#include <climits>
#include <bitset>
#include <list>
#include <random>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())

using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;

// solve
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}

int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, -1, 0, 1};

int m[101][101];
bool used[101][101];
int w, h;

bool dfs(int i, int j, int i1, int j1){
  used[i][j] = true;
  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 && m[i][j] == m[nx][ny]){
      if(nx == i1 && ny == j1)continue;
      if(used[nx][ny])return true;
      if(dfs(nx, ny, i, j))return true;
    }
  }
  return false;
}

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> w >> h;
  rep(i, h){
    rep(j, w){
      cin >> m[i][j];
    }
  }
  for(int i = 0; i < h; i++){
    for(int j = 0; j < w; j++){
      if(!used[i][j]){
        if(dfs(i, j, -1, -1)){
          cout << "possible" << endl;
          return 0;
        }
      }
    }
  }
  cout << "impossible" << endl;
	return 0;
}