#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
typedef long long ll;
static const int INF = 1e8;

int done[110][110];
int a[110][110];
int dx[] = {-1,0,1,0};
int dy[] = {0,1,0,-1};
int H,W;
int ok = 0;
int dfs(int x,int y,int px,int py){
	if(done[y][x]) return 0;
	done[y][x] = 1;
	for(int i = 0;i < 4; i++){
		int nx = x + dx[i];
		int ny = y + dy[i];
		if(nx == px && ny == py) continue;
		if(a[ny][nx] != a[y][x] ) continue;
		if(done[ny][nx]) ok = 1;
		dfs(nx,ny,x,y);
	}
}

int main(){
	cin >> W >> H;
	for(int i = 1;i <= H; i++){
		for(int j=1; j <= W; j++){
			cin >> a[i][j];
		}
	}
	for(int i = 1; i <= H;i++){
		for(int j = 1 ; j <=W ; j++){
			dfs(j,i,-1,-1);
		}
	}
	cout << (ok?"possible":"impossible") << endl;
}