#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef unsigned long long ULL;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(auto i: c)
#define SORT(c) sort((c).begin(),(c).end())

const double EPS = 1e-10;
const double PI  = acos(-1.0);

#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	struct pos{
		int x;
		int y;

	};
	const int dir[4][2] = {
			{0, 1},
			{1, 0},
			{0, -1},
			{-1, 0}
	};

	int W, H;
	cin >> W >> H;
	VVI M(H, VI(W));
	vector<pos> s(1001);
	REP(i, H) REP(j, W){
		cin >> M[i][j];
		s[M[i][j]] = {i, j};
	}

	bool ret = false;
	REP(i, 1001){
		queue<pos> que;
		vector<vector<bool> > bM(H, vector<bool>(W, false));
		que.push(s[i]);

		while(!que.empty()){
			pos tmp = que.front();
			que.pop();
			int cX = tmp.x, cY = tmp.y;
			bM[cX][cY] = true;
			int occupied = 0;
			REP(k, 4){
				int nX = cX + dir[k][0], nY = cY + dir[k][1];
				if(nX < 0 || nX >= H || nY < 0 || nY >= W) continue;
				if(M[nX][nY] == i){
					if(bM[nX][nY] == true) occupied++;
					else{
						pos next = {nX, nY};
						que.push(next);
					}
				}
			}
			if(occupied >= 2){
				ret = true;
				break;
			}
		}
		if(ret) break;
	}

	if(ret) cout <<"possible" << endl;
	else cout << "impossible" << endl;



	return 0;
}