#include"bits/stdc++.h"

//#include<bits/stdc++.h>
using namespace std;
#define print(x) cout<<x<<endl;
#define rep(i,a,b) for(int i=a;i<b;i++)
typedef long long ll;
const ll mod = 10000000000;

int h, w;
int m[102][102] = {};
int flag[102][102] = {};
int dx[4] = { 0,1,0,-1 };
int dy[4] = { 1,0,-1,0 };

bool dfs(int y, int x, int pre) {
	flag[y][x] = 1;
	rep(k, 0, 4) {
		int nx = x + dx[k];
		int ny = y + dy[k];
		if (pre == k)continue;
		if (!(nx >= 0 && nx < w&&ny >= 0 && ny < h))continue;
		if (m[ny][nx] != m[y][x])continue;
		if (flag[ny][nx])return 1;
		if (dfs(ny, nx, (k + 2) % 4))return 1;
	}
	return 0;
}

int main() {
	cin >> w >> h;
	rep(i, 0, h) {
		rep(j, 0, w) {
			cin >> m[i][j];
		}
	}
	rep(i, 0, h) {
		rep(j, 0, w) {
			if (dfs(i, j, -1)) {
				if (flag[i][j])continue;
				print("possible");
				return 0;
			}
		}
	}
	print("impossible"); 
	return 0;
}