#include <iostream>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>	// require sort next_permutation count __gcd reverse etc.
#include <cstdlib>	// require abs exit atof atoi 
#include <cstdio>		// require scanf printf
#include <functional>
#include <numeric>	// require accumulate
#include <cmath>		// require fabs
#include <climits>
#include <limits>
#include <cfloat>
#include <iomanip>	// require setw
#include <sstream>	// require stringstream 
#include <cstring>	// require memset
#include <cctype>		// require tolower, toupper
#include <fstream>	// require freopen
#include <ctime>		// require srand
#define rep(i,n) for(int i=0;i<(n);i++)
#define ALL(A) A.begin(), A.end()
/*
	No.13 囲みたい！

	幅優先探索
*/
using namespace std;

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

const int dr[] = {-1, 0, 1, 0 };
const int dc[] = { 0, 1, 0,-1 };

const int MAX_W = 105;
const int MAX_H = 105;
const int MAX_M = 1005;

int field[MAX_W][MAX_H];
int visit[MAX_W][MAX_H];

int W, H;

void disp (void ){
	cerr << endl;
	rep (i, H ){
		rep (j, W ){
			cerr << setw(2) << field[i][j];
		} // end rep
		cerr << endl;
	} // end rep
}

void disp_visit (void ){
	cerr << endl;
	rep (i, H ){
		rep (j, W ){
			cerr << setw(2) << visit[i][j];
		} // end rep
		cerr << endl;
	} // end rep
}


bool bfs (int row, int col, int num ){

	visit[row][col] = 0;
	queue<P> que;
	que.push (P (row, col ) );

	while (!que.empty() ){
		P curr = que.front(); que.pop();
		int cr = curr.first;
		int cc = curr.second;
		rep (k, 4 ){
			int nr = cr + dr[k];
			int nc = cc + dc[k];
			if (nr < 0 || nr >= H || nc < 0 || nc >= W ) continue;
			if (field[nr][nc] != num ) continue;

			if (visit[nr][nc] == -1 ){
				que.push (P (nr, nc ) );
				visit[nr][nc] = visit[cr][cc] + 1;
			}else{
				int diff = visit[nr][nc] - visit[cr][cc];
				if (diff >= 1 ) return true;
			} // end if
		} // end rep
	} // end while

//	disp_visit ();

	return false;
}

int main()
{
	memset (field, 0, sizeof (field ) );
	memset (visit,-1, sizeof (visit ) );

	ios_base::sync_with_stdio(0);
	cin >> W >> H;
	rep (i, H ) rep (j, W ) cin >> field[i][j];

//	disp ();

	bool res = false;
	rep (i, H ){
		rep (j, W ){
			int curr = field[i][j];
			if (visit[i][j] == -1 ){
				res |= bfs (i, j, curr );
			} // end if
		} // end rep
	} // end rep

	cout << (res ? "possible" : "impossible" ) << endl;	
	return 0;
}