#include <bits/stdc++.h>

using namespace std;

#define REP(i, n)   for(int i = 0; i < (n); i++)
#define REPS(i, n)  for(int i = 1; i <= (n); i++)
#define RREP(i, n)  for(int i = (n)-1; i >= 0; i--)
#define RREPS(i, n) for(int i = (n); i > 0; i--)
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define mp make_pair
#define mt make_tuple
#define pb push_back

using ll  = long long;
using pi  = pair<int, int>;
using pl  = pair<ll, ll>;
using vi  = vector<int>;
using vl  = vector<ll>;
using vs  = vector<string>;
using vb  = vector<bool>;
using vvi = vector<vi>;

const int MOD = 1e9 + 7;
const int INF = 1e9 + 7;
const ll  INFL = 1e18;
const double PI = 3.141592653589793;
const double EPS = 1e-9;

template<class T> bool chmax(T &a, const T &b) { if(a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if(a > b) { a = b; return true; } return false; }

int W, H;
vvi G;
int dh[] = {1, 0, 0, -1};
int dw[] = {0, 1, -1, 0};
vector<vb> v;

bool dfs(int h, int w, int n, int oh, int ow)
{
    v[h][w] = true; 
    REP(i, 4)
    {
        int nh = h + dh[i];
        int nw = w + dw[i];
        if(0 <= nh && nh < H && 0 <= nw && nw < W && G[nh][nw] == n)
        {
            if(nh == oh && nw == ow) continue;
            if(v[nh][nw]) return true;
            return dfs(nh, nw, n, h, w);
        }
    }
    return false;
}

signed main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);

    cin >> W >> H;
    G.resize(H, vi(W));
    v.resize(H, vb(W, false));
    REP(i, H) REP(j, W) cin >> G[i][j];
    REP(i, H)
    {
        REP(j, W)
        {
            if(!v[i][j] && dfs(i, j, G[i][j], -1, -1))
            {
                cout << "possible" << endl;
                return 0;
            }
        }
    }
    cout << "impossible" << endl;
}