#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <complex>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <bitset>
#include <ctime>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <numeric>
#include <tuple>
#include <sstream>
#include <fstream>
 
using namespace std;
#define REP(i, n) for (int (i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int (i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for(int (i) = (a) - 1; (i) >= 0; (i)--)
#define FORR(i, a, b) for(int (i) = (a) - 1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template<typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define FIND(s, e) ((s).find(e) != (s).end())
#define EB emplace_back
 
const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;

int H, W;
VVI v;
VT<VT<bool>> vis;
const int vh[] = {0, 0, -1, 1};
const int vw[] = {-1, 1, 0, 0};
bool inside(int h, int w) {
    return 0 <= h && h < H && 0 <= w && w < W;
}

int main(void) {
    scanf("%d%d", &W, &H);
    vis.resize(H, VT<bool>(W, false));
    v.resize(H, VI(W));
    REP(i, H) REP(j, W) {
        scanf("%d", &v[i][j]);
    }
    queue<pair<PII, PII>> q;//(now, prev)
    bool f = false;
    REP(i, H) REP(j, W) if (!vis[i][j]) {
        const int num = v[i][j];
        q.push(MP(MP(i, j), MP(-1, -1)));
        vis[i][j] = true;
        while (!q.empty()) {
            auto now = q.front().first;
            auto prev = q.front().second;
            q.pop();
            REP(i, 4) {
                const int nh = now.first + vh[i];
                const int nw = now.second + vw[i];
                if (!inside(nh, nw)) continue;
                if (v[nh][nw] != num) continue;
                //cout << now.first << " " << now.second << " " << nh << " " << nw << " " << v[nh][nw] << endl;
                if (vis[nh][nw]) {
                    if (MP(nh, nw) == prev) continue;
                    f = true;
                    //cout << nh << " " << nw << endl;
                    goto last;
                } else vis[nh][nw] = true;
                
                q.push(MP(MP(nh, nw), now));
            }
        }
    }
    last:
    cout << (f ? "possible" : "impossible") << endl;
}