#include <bits/stdc++.h>

using namespace std;

#define all(c) ((c).begin()), ((c).end())
#define dump(c) cerr << "> " << #c << " = " << (c) << endl;
#define iter(c) __typeof((c).begin())
#define tr(i, c) for (iter(c) i = (c).begin(); i != (c).end(); i++)
#define REP(i, a, b) for (int i = a; i < (int)(b); i++)
#define rep(i, n) REP(i, 0, n)
#define mp make_pair
#define fst first
#define snd second
#define pb push_back
#define debug(fmt, ...) \
        fprintf( stderr, \
                  fmt "\n", \
                  ##__VA_ARGS__ \
        )

typedef unsigned int uint;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int INF = 1 << 29;
const double EPS = 1e-10;

double zero(double d) {
    return d < EPS ? 0.0 : d;
}

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 (b < a) {
        a = b;
        return true;
    }
    return false;
}

#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
    return os << '(' << p.first << ',' << p.second << ')';
}

template<typename T>
ostream &operator<<(ostream &os, const vector<T> &a) {
    os << '[';
    rep(i, a.size()) os << (i ? " " : "") << a[i];
    return os << ']';
}

string toString(int i) {
    stringstream ss;
    ss << i;
    return ss.str();
}

const int MOD = 1000000007;

// a^k
ll fpow(ll a, ll k, int M) {
    ll res = 1ll;
    ll x = a;
    while (k != 0) {
        if ((k & 1) == 1)
            res = (res * x) % M;
        x = (x * x) % M;
        k >>= 1;
    }
    return res;
}

struct prepare {
    prepare() {
        cout.setf(ios::fixed, ios::floatfield);
        cout.precision(8);
        ios_base::sync_with_stdio(false);
    }
} _prepare;

int W, H;
int nums[101][101];
int vis[101][101];

int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};

bool dfs(int sr, int sc, int d) {
    vis[sr][sc] = d;

    rep(i, 4) {
        int nr = sr + dy[i];
        int nc = sc + dx[i];

        if (0 <= nr && nr < H && 0 <= nc && nc < W &&
            nums[sr][sc] == nums[nr][nc]) {
            if(vis[nr][nc] != -1 && d - vis[nr][nc] > 1)
                return true;
            else if (vis[nr][nc] == -1) {
                if(dfs(nr, nc, d+1))
                    return true;
            }
        }
    }
    return false;
}

int main() {
    cin >> W >> H;
    rep(i, H)
        rep(j, W)
            cin >> nums[i][j];

    rep(r, H-1) {
        rep(c, W-1) {
            if(nums[r][c] == nums[r+1][c] &&
               nums[r+1][c] == nums[r][c+1] &&
               nums[r][c+1] == nums[r+1][c+1]) {
                cout << "possible" << endl;
                return 0;
            }
        }
    }

    REP(i, 1, 1000 + 1) {
        memset(vis, -1, sizeof(vis));
        rep(r, H) {
            rep(c, W) {
                if (nums[r][c] == i && vis[r][c] == -1) {
                    if (dfs(r, c, 0)) {
                        cout << "possible" << endl;
                        return 0;
                    }
                }
            }
        }
    }

    cout << "impossible" << endl;
    return 0;
}