#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int W,H; cin >> W >> H;
    vector<vector<int>> M(H, vector<int>(W));
    rep(i,H)rep(j,W) cin >> M[i][j];

    int di[] = {0, 1, 0, -1};
    int dj[] = {1, 0, -1, 0};
    vector<vector<int>> used(H, vector<int>(W, 0));
    function<void(int,int,int,int)> dfs = [&](int i, int j, int pi, int pj) -> void {
        used[i][j] = 1;
        rep(d,4) {
            int ni = i + di[d], nj = j + dj[d];
            if(0 <= ni && ni < H && 0 <= nj && nj < W && M[i][j] == M[ni][nj] && make_pair(ni, nj) != make_pair(pi, pj)) {
                if(used[ni][nj]) {
                    cout << "possible" << endl;
                    exit(0);
                } else {
                    dfs(ni, nj, i, j);
                }
            }
        }
    };

    vector<int> vis(1001, 0);
    rep(i,H)rep(j,W) if(!vis[M[i][j]]) {
        vis[M[i][j]] = 1;
        dfs(i, j, -1, -1);
    }
    cout << "impossible" << endl;
    return 0;
}