#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

using ll = long long;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

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

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

    int W, H;
    cin >> W >> H;
    vector<vector<int>> M(H, vector<int>(W));
    for(int i = 0; i < H; ++i){
        for(int j = 0; j < W; ++j){
            cin >> M[i][j];
        }
    }
    vector<vector<bool>> visited(H, vector<bool>(W));
    auto dfs = [&](auto&& self, int x, int y, int px = -1, int py = -1) -> void {
        visited[x][y] = true;
        for(int i = 0; i < 4; ++i){
            int nx = x + dx[i], ny = y + dy[i];
            if(nx < 0 || nx >= H || ny < 0 || ny >= W)  continue;
            if(nx == px && ny == py)    continue;
            if(M[nx][ny] != M[x][y])    continue;
            if(visited[nx][ny]){
                cout << "possible\n";
                exit(0);
            }
            self(self, nx, ny, x, y);
        }
    };
    for(int x = 0; x < H; ++x){
        for(int y = 0; y < W; ++y){
            if(!visited[x][y])  dfs(dfs, x, y);
        }
    }
    cout << "impossible\n";

    return 0;
}