#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <time.h>
 
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define FFOR(i,a,b) for(int i=(a);i<=(b);i++)
#define REP(i,b) FOR(i,0,b)
#define RREP(i,b) FFOR(i,1,b)
#define PB push_back
#define F first
#define S second
#define BE(c) c.begin(),c.end()
using namespace std;
typedef long long LL;
typedef LL ut;
typedef long double ld;
typedef pair<ut,ut> pr;
typedef vector<pr> Vpr;
typedef vector<ut> VI;
typedef pair<ut,pr> ppr;
typedef priority_queue<pr,Vpr, greater<pr> > PQ;
const int SIZE=1e+5 + 1;
const ut INF=1<<30;
const ld eps=1e-6;
const LL mod=1e+9 + 7;

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

int W, H;
int maze[128][128];
bool visited[128][128];

bool check(int x, int y){
	return x >= 0 && x < W && y >= 0 && y < H; 	
}

bool dfs(int x, int y, int prev){
	visited[y][x] = true;
	REP(i,4){
		if(i == prev)
			continue;
		
		int nx = x + dx[i], ny = y + dy[i];
		if(!check(nx, ny))
			continue;
		if(maze[y][x] != maze[ny][nx])
			continue;
		if(visited[ny][nx])
			return true;
		if(dfs(nx, ny, (i + 2) % 4))
			return true;
	}
	return false;
}

int main() {
	cin >> W >> H;
	REP(i,H)REP(j,W)
		scanf("%d", &maze[i][j]);
	REP(i,H)REP(j,W){
		if(!visited[i][j]){
			if(dfs(j, i, -1)){
				cout << "possible" << endl;
				exit(0);
			}
		}
	}
	cout << "impossible" << endl;
	return 0;
}