#include<iostream>
#include <fstream>
#include <stdio.h>
#include <stdlib.h>
#define _USE_MATH_DEFINES
#include <math.h>
#include<string>
#include<vector>
#include<cmath>
#include<stack>
#include<queue>
#include<sstream>
#include<algorithm>
#include<map>
#include<complex>
#include<ctime>
#include<set>
using namespace std;


#define li			long long int
#define rep(i,to)	for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)	for(li i=(li)(start);i<((li)(to));i++)
#define pb			push_back
#define sz(v)		((li)(v).size())
#define bgn(v)		((v).begin())
#define eend(v)		((v).end())
#define allof(v)	(v).begin(), (v).end()
#define dodp(v,n)		memset(v,(li)n,sizeof(v))
#define bit(n)		(1ll<<(li)(n))
#define mp(a,b)		make_pair(a,b)
#define rin	rep(i,n)
#define EPS 1e-10
#define ETOL 1e-8
#define MOD 1000000007
#define F first
#define S second
#define p2(a,b)		cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)		cout<<a<<"\t"<<b<<"\t"<<c<<endl

li m[110][110];
li visit[110][110];

li dx[4]={-1, 1, 0, 0};
li dy[4]={0, 0, -1, 1};
	li w,h;

bool dfs(li y, li x, li num){
	visit[y][x]=num;
	rep(i,4){
		li nx=x+dx[i], ny=y+dy[i];
		if(min(nx, ny)>=0 && nx<w && ny<h && m[y][x]==m[ny][nx]){
			if(visit[ny][nx]>=0 && visit[y][x]-visit[ny][nx]!=1){
				return true;
			}
			if(visit[ny][nx]<0 && dfs(ny, nx, num+1))return true;
		}
	}
	return false;
}

int main(){
	cin>>w>>h;
	rep(i,h){
		rep(j,w){
			cin>>m[i][j];
			visit[i][j]=-1;
		}
	}


	bool res=false;
	rep(i,h){
		rep(j,w){
			if(visit[i][j]<0){
				//p2(res, m[i][j]);
				res=res||dfs(i,j,10);
			}
		}
	}

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


	return 0;
}