#include <bits/stdc++.h>
using namespace std;

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 tx = x + dx[i];
		int ty = y + dy[i];
		if( tx == px && ty == py ) continue;
		if( a[ty][tx] != a[y][x] ) continue;
		if( done[ty][tx] ) ok = 1;
		dfs(tx,ty,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;
	
}