#include <bits/stdc++.h>
using namespace std;
using i64 = int64_t;
using vi = vector<i64>;
using vvi = vector<vi>;

int main() {
    int w, h;
    cin >> w >> h;
    vvi b(h, vi(w));
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            cin >> b[i][j];
        }
    }
    
    auto id = [&](int i, int j) {
        return i * w + j;
    };
    auto valid = [&](int i, int j) {
        return 0 <= i && i < h && 0 <= j && j < w;
    };
    using ii = pair<int, int>;
    auto decode = [&](int i) {
        return ii(i / w, i % w);
    };
    
    
    int ok = 0;
    vvi visited(h, vi(w, 0));
    int dx[] = {-1, 1, 0, 0};
    int dy[] = {0, 0, -1, 1};
    function<void(int, int)> dfs = [&](int v, int p) {
        int x = decode(v).first;
        int y = decode(v).second;
        if (visited[x][y]) {
            ok = 1;
            return;
        }
        visited[x][y] = 1;
        for (int k = 0; k < 4; k++) {
            int xx = x + dx[k];
            int yy = y + dy[k];
            if (valid(xx, yy) && id(xx, yy) != p && b[xx][yy] == b[x][y]) {
                dfs(id(xx, yy), v);
            }
        }
    };
    
    for (int i = 0; i < h; i++) {
        for (int j = 0; j < w; j++) {
            ok = 0;
            visited = vvi(h, vi(w));
            dfs(id(i, j), -1);
            if (ok) {
                cout << "possible" << endl;
                return 0;
            }
        }
    }
    cout << "impossible" << endl;
}