#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <string>
#include <queue>
#include <map>
#include <bitset>
#include <set>
#include <stack>
#include <numeric>
#include <unordered_map>
#include <random>

using namespace std;

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vd = vector<double>;
using vs = vector<string>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;
using vpii = vector<pii>;
using vpll = vector<pll>;
using vpdd = vector<pdd>;
const int inf = (1 << 30) - 1;
const ll INF = 1LL << 60;
//const int MOD = 1000000007;
const int MOD = 998244353;

int dy[4] = { 0, 1, 0, -1 };
int dx[4] = { 1, 0, -1, 0 };

bool dfs(vvi& a, vvb& visit, int y, int x, int dir, int h, int w) {
	visit[y][x] = true;
	bool ret = false;
	for (int i = 0; i < 4; i++) {
		if (i == (dir + 2) % 4) continue;
		
		int x2 = x + dx[i];
		int y2 = y + dy[i];
		if (y2 < 0 || y2 > h - 1 || x2 < 0 || x2 > w - 1) continue;
		if (a[y2][x2] != a[y][x]) continue;

		if (visit[y2][x2]) return true;
		
		if (!visit[y2][x2]) {
			ret = dfs(a, visit, y2, x2, i, h, w);
			if (ret) return true;
		}
	}
	return false;
}

int main() {
	int h, w;
	cin >> w >> h;
	vvi a(h, vi(w));
	for (int y = 0; y < h; y++) {
		for (int x = 0; x < w; x++) {
			cin >> a[y][x];
		}
	}
	vvb visit(h, vb(w, false));
	for (int y = 0; y < h; y++) {
		for (int x = 0; x < w; x++) {
			if (visit[y][x]) continue;
			if (dfs(a, visit, y, x, -11, h, w)) {
				cout << "possible" << endl;
				return 0;
			}
		}
	}
	cout << "impossible" << endl;
	
	return 0;
}