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

int H, W;
int board[100][100];
bool check[100][100];

bool ok(int y, int x){
	return y >= 0 && x >= 0 && y < H && x < W;
}

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

bool dfs(int y, int x, int pre){
	check[y][x] = true;
	for (int k = 0; k < 4; k++)
	{
		if (pre == k) continue;
		int ny = y + dy[k];
		int nx = x + dx[k];
		if (!ok(ny, nx)) continue;
		if (board[y][x] != board[ny][nx]) continue;
		if (check[ny][nx]) return true;
		if (dfs(ny, nx, (k + 2) % 4)) return true;
	}
	return false;
}

int main() {
	cin >> W >> H;
	for (int i = 0; i < H; i++)
	{
		for (int j = 0; j < W; j++)
		{
			cin >> board[i][j];
		}
	}
	for (int i = 0; i < H; i++)
	{
		for (int j = 0; j < W; j++)
		{
			if (check[i][j]) continue;
			if (dfs(i, j, -1)){
				cout << "possible" << endl;
				return 0;
			}
		}
	}
	cout << "impossible" << endl;
}