#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    using namespace std;
    int w, h, max_val = 0;
    cin >> w >> h;
    vector m(h, vector<int>(w));
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            cin >> m[i][j];
            max_val = max(max_val, m[i][j]);
        }
    }

    constexpr int di[] = {-1, 0, 1, 0};
    constexpr int dj[] = {0, -1, 0, 1};
    auto calc_next = [&](int p) -> vector<int> {
        int i = p / w, j = p % w;
        vector<int> res;
        for (int k = 0; k < 4; k++) {
            int ni = i + di[k], nj = j + dj[k];
            if (ni < 0 || ni >= h || nj < 0 || nj >= w) { continue; }
            if (m[i][j] == m[ni][nj]) { res.push_back(ni * w + nj); }
        }
        return res;
    };

    for (int x = 1; x <= max_val; x++) {
        vector<bool> vis(h * w, false);
        for (int i = 0; i < h; i++) {
            for (int j = 0; j < w; j++) {
                int p = i * w + j;
                if (m[i][j] != x || vis[p]) { continue; }
                auto dfs = [&](auto dfs, int v, int p) -> void {
                    for (int nv : calc_next(v)) {
                        if (nv == p) { continue; }
                        if (vis[nv]) {
                            cout << "possible\n";
                            exit(0);
                        }
                        vis[nv] = true;
                        dfs(dfs, nv, v);
                    }
                };
                dfs(dfs, p, -1);
            }
        }
    }
    cout << "impossible\n";
}