#include <cstdio>
using namespace std;

#define W_MAX 100
#define H_MAX 100

int W, H;
int field[H_MAX][W_MAX];
bool used[H_MAX][W_MAX];
const int dy[] = {-1, 0, 1, 0};
const int dx[] = {0, 1, 0, -1};

bool dfs(int y, int x, const int& M) {
	used[y][x] = 1;
	int cnt = 0;
	for (int i = 0; i < 4; i++) {
		int ny = y + dy[i];
		int nx = x + dx[i];
		if (ny < 0 || nx < 0 || ny >= H || nx >= W) continue;
		if (used[ny][nx] || field[ny][nx] != M) {
			if (field[ny][nx] == M) cnt++;
			continue;
		}
		if (dfs(ny, nx, M)) return 1;
	}
	if (1 < cnt) return 1;
	return 0;
}

int main() {
	scanf("%d%d", &W, &H);
	for (int i = 0; i < H; i++) {
		for (int j = 0; j < W; j++) {
			scanf("%d", &field[i][j]);
		}
	}
	
	bool f = 0;
	for (int i = 0; i < H; i++) {
		for (int j = 0; j < W; j++) {
			if (used[i][j]) continue;
			if (dfs(i, j, field[i][j])) {
				f = 1;
				break;
			}
		}
		if (f) break;
	}
	
	printf("%s\n", f ? "possible" : "impossible");
	
	return 0;
}