#include <bits/stdc++.h>

#define REP(i, x, n) for(int i = x; i < (int)(n); i++)
#define rep(i, n) REP(i, 0, n)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define F first
#define S second
#define mp make_pair

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;

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

int W, H;
int M[100][100];
bool visited[100][100];
bool found;

void rec(int y, int x, int py, int px, int n) {
  if(found) return;
  
  visited[y][x] = true;

  rep(i, 4) {
    int ny = y + dy[i];
    int nx = x + dx[i];
    if(!(0 <= ny && ny < H && 0 <= nx && nx < W)) continue;
    if(ny == py && nx == px) continue;
    if(M[ny][nx] != n) continue;
    if(visited[ny][nx]) {
      found = true;
      return;
    }
    rec(ny, nx, y, x, n);
  }
}

int main() {
  // ios_base::sync_with_stdio(false);
  cin >> W >> H;
  rep(i, H) rep(j, W) cin >> M[i][j];
  rep(i, H) rep(j, W) {
    if(found) break;
    if(visited[i][j]) continue;
    rec(i, j, i, j, M[i][j]);
  }
  cout << (found ? "possible" : "impossible") << endl;
  return 0;
}