#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;

int W, H;
int field[105][105];
vector<int> G[10005];

int vx[4] = {0,0,1,-1};
int vy[4] = {1,-1,0,0};
int xx[1005];

bool check(int s){
  int N = W*H;
  vector<int> visited(N, 0);
  queue< pair<int,int> > que;
  que.push(make_pair(s, -1));
  while(!que.empty()){
    pair<int,int> p = que.front(); que.pop();
    visited[p.first] = 1;
    rep(i,G[p.first].size()){
      if(G[p.first][i] == p.second) continue;
      if(visited[G[p.first][i]] == 1) return true;
      que.push(make_pair(G[p.first][i], p.first));
    }
  }
  return false;
}

int main(){
  cin >> W >> H;

  rep(i,H){
    rep(j,W){
      cin >> field[i][j];
    }
  }

  rep(i,1005){
    xx[i] = -1;
  }

  rep(i,H){
    rep(j,W){
      xx[field[i][j]] = i*W+j;
      rep(k,4){
        int nx = j+vx[k], ny = i+vy[k];
        if(0<=nx && nx<W && 0<=ny && ny<H){
          if(field[i][j] == field[ny][nx]){
            G[i*W+j].push_back(ny*W+nx);
            G[ny*W+nx].push_back(i*W+j);
          }
        }
      }
    }
  }

  rep(i,1005){
    if(xx[i] >= 0){
      if(check(xx[i])){
        cout << "possible" << endl;
        return 0;
      }
    }
  }
  cout << "impossible" << endl;
  return 0;
}