#define _USE_MATH_DEFINES

#include <iostream>
#include <iomanip>

#include <algorithm>
#include <cmath>

#include <string>
#include <array>
#include <list>
#include <queue>
#include <vector>
#include <complex>
#include <set>
#include <map>

/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;

#define PII pair<int,int>
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////

int main(void){
    std::cin.tie(0); 
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed;//
    cout << setprecision(10);//

	int W,H;
	cin>>W>>H;
	int field[100][100];
	bool usefield[100][100];
	rep(j,H)rep(i,W){
		cin>>field[i][j];
		usefield[i][j] = false;
	}

	queue< pair<int,pair<int,int> > > que;
	bool useNum[1001];
	rep(i,1001)useNum[i] = false;
	int dx[4]={1,0,-1,0};
	int dy[4]={0,1,0,-1};

for(int wi=0;wi<W;++wi){
for(int hi=0;hi<H;++hi){
if( usefield[wi][hi] == false){
	{
		usefield[wi][hi] = true;
		int terNum = field[wi][hi];
		set< pair<int,int> > hit;
		hit.insert( pair<int,int>(wi,hi) );
		for(int d=0;d<4;++d){
			if(0 <= wi+dx[d] && wi+dx[d] < W && 0 <= hi+dy[d] && hi+dy[d] < H){
				if( terNum == field[ wi+dx[d] ][ hi+dy[d] ] ){
					usefield[ wi+dx[d] ][ hi+dy[d] ] = true;
					hit.insert( pair<int,int>(wi+dx[d],hi+dy[d]) );
					que.push( pair<int,pair<int,int> >(d,pair<int,int>(wi+dx[d],hi+dy[d]) ) );
				}
			}
			
		}
		////////////////
		pair<int,pair<int,int> > tque;
		int tx,ty;
		while( !que.empty() ){
			tque = que.front();
			tx = tque.second.first;
			ty = tque.second.second;
			que.pop();
			for(int d=0;d<4;++d){
				if( (tque.first+2)%4 != d &&
					0 <= tx+dx[d] && tx+dx[d] < W && 0 <= ty+dy[d] && ty+dy[d] < H){
					if( terNum == field[ tx+dx[d] ][ ty+dy[d] ] ){
						usefield[ tx+dx[d] ][ ty+dy[d] ] = true;
						if(hit.insert( pair<int,int>(tx+dx[d],ty+dy[d]) ).second == false ){
							P("possible");
							return 0;
						}
						que.push( pair<int,pair<int,int> >(d,pair<int,int>(tx+dx[d],ty+dy[d]) ) );
					}
				}
			}
		}
	}
}}}
	P("impossible");
    return 0;
}