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

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

void dfs(int x,int y,int px,int py,int color){
	visited[x][y] = color;
	for(int i = 0;i<4;i++){
		int nx = x+dx[i],ny = y+dy[i];
		if(nx<0||nx>=h||ny<0||ny>=w)continue;
		if(nx==px&&ny==py)continue;
		if(G[x][y]==G[nx][ny]){
			if(visited[nx][ny]==0){
				dfs(nx,ny,x,y,color);
			}else{
				is = true;
			}
		}
	}
}

void solve(){
	stack<P> Q;
	stack<P> Qp;
	is = false;
	visited = vector<vector<int>>(h,vector<int>(w));
	for(int i = 0;i<h;i++){
		for(int j = 0;j<w;j++){
			if(visited[i][j]==0){
				dfs(i,j,-1,-1,-1);
			}
		}
	}

	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();
}