#include <bits/stdc++.h>
using namespace std;

vector<vector<int>> G;

void dfs(int v, int pre, int s) {
  for (auto next : G.at(v)) {
    if (next == pre) continue;
    if (next == s) {
      cout << "possible" << "\n";
      exit(0);
    }
    dfs(next, v, s);
  } 
}

int main() {
  int W, H, N = 0;
  cin >> W >> H;
  vector<vector<pair<int, int>>> M(H, vector<pair<int, int>>(W));
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      int m;
      cin >> m;
      M.at(i).at(j) = {m, N++};
    }
  }
  G.resize(N);
  vector<int> mh(4), mw(4);
  mh = {-1, 0, 1, 0};
  mw = {0, 1, 0, -1};
  for (int h = 0; h < H; h++) {
    for (int w = 0; w < W; w++) {
      for (int i = 0; i < 4; i++) {
        int nh = h + mh.at(i);
        int nw = w + mw.at(i);
        if (nh < 0 || nh >= H || nw < 0 || nw >= W) continue;
        auto [a, b] = M.at(h).at(w);
        auto [c, d] = M.at(nh).at(nw);
        if (a == c) G.at(b).push_back(d);
      }
    }
  }
  for (int i = 0; i < N; i++) dfs(i, -1, i);
  cout << "impossible" << "\n";
}