#include<bits/stdc++.h>
using namespace std;
#include<atcoder/all>
using namespace atcoder;
using ll = long long;
int h,w;
vector<vector<int>> G;
using P = pair<int,int>;

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

void solve(){
	queue<P> Q;
	queue<P> Qp;
	bool is = false;
	vector<vector<int>> visited(h,vector<int>(w));
	for(int i = 0;i<h;i++){
		for(int j = 0;j<w;j++){
			if(visited[i][j])continue;
			int curIdx = i*w+j+1;
			visited[i][j] = curIdx;
			Q.emplace(i,j);
			Qp.emplace(-1,-1);
			while(Q.size()){
				auto [x,y] = Q.front();
				auto [px,py] = Qp.front();
				//if(px!=-1)cout<<G[x][y]<<' '<<G[px][py]<<endl;
				//cout<<x<<' '<<y<<endl;
				Q.pop();
				Qp.pop();
				for(int k = 0;k<4;k++){
					int nx = x+dx[k],ny = y+dy[k];
					if(nx<0||nx>=h||ny<0||ny>=w)continue;
					if(G[x][y]==G[nx][ny]){
						if(visited[nx][ny]==curIdx&&nx!=px&&ny!=py){
							//cout<<nx<<' '<<ny<<' '<<px<<' '<<py<<endl;
							is = true;
						}
						if(visited[nx][ny]==0){
							visited[nx][ny] = curIdx;
							Q.emplace(nx,ny);
							Qp.emplace(x,y);
						}
					}
				}
			}
		}
	}
	/*
	for(int i = 0;i<h;i++){
		for(int j = 0;j<w;j++){
			cout<<visited[i][j]<<' ';
		}
		cout<<endl;
	}
	*/
	if(is)cout<<"possible"<<endl;
	else cout<<"impossible"<<endl;
}

signed main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	cin >> w >> h;
	G = vector<vector<int>>(h,vector<int>(w));
	for(int i = 0;i<h;i++){
		for(int j = 0;j<w;j++){
			cin >> G[i][j];
		}
	}
	solve();
}