#include <bits/stdc++.h>

using namespace std;

typedef pair<int,int> P;
typedef pair<int,pair<int,int>> PP;
typedef long long ll;

const double EPS = 1e-8;
const int INF = 1e9;
const int MOD = 1e9+7;

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

bool dfs(const int mm,int y,int x,int prevy,int prevx,vector<vector<int>>& m,vector<vector<int>> used){
	int ny,nx;
	if(used[y][x] == 1)return true;
	used[y][x] = 1;
	for(int i=0;i<4;i++){
		ny = y + dy[i];nx = x + dx[i];
		if(ny < 0 || nx < 0 || ny > m.size()-1 || nx > m[0].size()-1)continue;
		if(ny == prevy && nx == prevx)continue;
		if(mm != m[ny][nx])continue;
		return dfs(mm,ny,nx,y,x,m,used);
	}
	return false;
}

int main(void) {
	int w,h;
	cin >> w >> h;
	vector<vector<int>> m(h,vector<int>(w));
	vector<vector<int>> used(h,vector<int>(w));
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			cin >> m[i][j];
		}
	}

	bool ans = false;
	for(int i=0;i<h;i++){
		for(int j=0;j<w;j++){
			if(used[i][j] != 0)continue;
			ans |= dfs(m[i][j],i,j,-1,-1,m,used);
		}
	}
	if(ans)cout << "possible" << endl;
	else cout << "impossible" << endl;

	return 0;
}