#include <bits/stdc++.h>
using namespace std;
vector<int> dy = {1, 0, -1, 0};
vector<int> dx = {0, 1, 0, -1};
int main(){
  int W, H;
  cin >> W >> H;
  vector<vector<int>> M(H, vector<int>(W));
  for (int i = 0; i < H; i++){
    for (int j = 0; j < W; j++){
      cin >> M[i][j];
    }
  }
  vector<vector<int>> id(H, vector<int>(W));
  for (int i = 0; i < H; i++){
    for (int j = 0; j < W; j++){
      id[i][j] = i * W + j;
    }
  }
  vector<vector<int>> E(H * W);
  for (int i = 0; i < H; i++){
    for (int j = 0; j < W; j++){
      for (int k = 0; k < 4; k++){
        int y = i + dy[k];
        int x = j + dx[k];
        if (0 <= y && y < H && 0 <= x && x < W){
          if (M[i][j] == M[y][x]){
            int v = id[i][j];
            int w = id[y][x];
            E[v].push_back(w);
          }
        }
      }
    }
  }
  vector<int> cc(H * W, -1);
  int cnt = 0;
  for (int i = 0; i < H * W; i++){
    if (cc[i] == -1){
      cc[i] = cnt;
      queue<int> Q;
      Q.push(i);
      while (!Q.empty()){
        int v = Q.front();
        Q.pop();
        for (int w : E[v]){
          if (cc[w] == -1){
            cc[w] = cc[v];
            Q.push(w);
          }
        }
      }
      cnt++;
    }
  }
  vector<int> vcnt(cnt, 0);
  vector<int> ecnt(cnt, 0);
  for (int i = 0; i < H * W; i++){
    vcnt[cc[i]]++;
    for (int j : E[i]){
      if (j > i){
        ecnt[cc[i]]++;
      }
    }
  }
  bool ok = false;
  for (int i = 0; i < cnt; i++){
    if (ecnt[i] >= vcnt[i]){
      ok = true;
    }
  }
  if (ok){
    cout << "possible" << endl;
  } else {
    cout << "impossible" << endl;
  }
}