#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <numeric>
#include <cctype>
#include <tuple>
#include <climits>
#include <bitset>
#include <cassert>
#include <random>

#ifdef _MSC_VER
#include <agents.h>
#endif

#define FOR(i, a, b) for(int i = (a); i < (int)(b); ++i)
#define rep(i, n) FOR(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define REV(v) v.rbegin(), v.rend()
#define MEMSET(v, s) memset(v, s, sizeof(v))
#define UNIQUE(v) (v).erase(unique(ALL(v)), (v).end())
#define MP make_pair
#define MT make_tuple

using namespace std;

typedef long long ll;
typedef pair<int, int> P;

const int N = 110;
int m[N][N];
int vis[N][N];

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

int w, h;

int dfs(int r, int c, int d){
	vis[r][c] = 1;

	for (int i = 0; i < 4; ++i){
		if (i == d) continue;
		int nr = r + dx[i], nc = c + dy[i];
		if (nr < 0 || nr >= h || nc < 0 || nc >= w) continue;
		if (m[nr][nc] != m[r][c]) continue;
		if (vis[nr][nc]) return true;
		else if(dfs(nr, nc, (i + 2) % 4)) return true;
	}
	return false;
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin >> w >> h;
	rep(i, h) rep(j, w) cin >> m[i][j];

	rep(i, h) rep(j, w){
		if (!vis[i][j] && dfs(i, j, -1)){
			cout << "possible" << endl;
			return 0;
		}
	}
	cout << "impossible" << endl;

	return 0;
}