#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001



int main(){
	
	int w,h;
	cin>>w>>h;
	
	vector a(h,vector<int>(w));
	rep(i,h){
		rep(j,w){
			cin>>a[i][j];
		}
	}
	
	dsu D(h*w);
	vector dx = {0,0,1,-1},dy = {1,-1,0,0};
	
	rep(i,h){
		rep(j,w){
			rep(k,4){
				int y = i+dy[k],x = j+dx[k];
				if(y<0||y>=h||x<0||x>=w)continue;
				if(a[i][j]==a[y][x]){
					D.merge(i*w+j,y*w+x);
				}
			}
		}
	}
	
	vector<vector<int>> ind(h,vector<int>(w));
	auto g = D.groups();
	
	rep(i,g.size()){
		rep(j,g[i].size()){
			ind[g[i][j]/w][g[i][j]%w] = i;
		}
	}
	
	vector<int> cnt(g.size(),0);
	
	rep(i,h){
		rep(j,w){
			rep(k,4){
				int y = i+dy[k],x = j+dx[k];
				if(y<0||y>=h||x<0||x>=w)continue;
				if(a[i][j]==a[y][x]){
					cnt[ind[i][j]]++;
				}
			}
		}
	}
	
	rep(i,g.size()){
		if(g[i].size()!=(cnt[i]>>1)+1){
			cout<<"possible"<<endl;
			return 0;
		}
	}
	cout<<"impossible"<<endl;
	
	return 0;
}