#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <functional>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cctype>
#include <string>
#include <cstring>

using namespace std;

int main() {
  int w, h;
  cin >> w >> h;
  int m[w][h];
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      cin >> m[j][i];
    }
  }

  int memo[w][h];
  memset(memo, -1, sizeof(memo));
  queue <int> qu;
  int dx[] = {-1, 0, 1, 0};
  int dy[] = {0, -1, 0, 1};
  bool ok = false;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      if (memo[j][i] == -1) {
	qu.push(j);
	qu.push(i);
	memo[j][i] = 0;
	while (!qu.empty()) {
	  int x = qu.front(); qu.pop();
	  int y = qu.front(); qu.pop();

	  for (int k = 0; k < 4; k++) {
	    int nx = x + dx[k];
	    int ny = y + dy[k];
	    if (nx < 0 || ny < 0 || nx >= w || ny >= h ||
		m[nx][ny] != m[x][y] ||
		(memo[nx][ny] != -1 && memo[nx][ny] == memo[x][y]-1)) {
	      continue;
	    }
	    if (memo[nx][ny] != -1) {
	      cout << "possible" << endl;
	      return 0;
	    }
	    memo[nx][ny] = memo[x][y]+1;
	    qu.push(nx);
	    qu.push(ny);
	  }
	}
      }
    }
  }

  cout << "impossible" << endl;
}