#include <bits/stdc++.h>

#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define RFOR(i, a, b) for (ll i = (b)-1; i >= (a); i--)
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define rep1(i, n) for (ll i = 1; i <= (n); i++)
#define rrep(i, n) for (ll i = (n)-1; i >= 0; i--)

#define pb push_back
#define mp make_pair
#define fst first
#define snd second
#define show(x) cout << #x << " = " << x << endl
#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
#define pii pair<int, int>
#define vi vector<int>

using namespace std;
template <class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& p)
{
    return o << "(" << p.first << "," << p.second << ")";
}
template <class T>
ostream& operator<<(ostream& o, const vector<T>& vc)
{
    o << "sz = " << vc.size() << endl
      << "[";
    for (const T& v : vc)
        o << v << ",";
    o << "]";
    return o;
}
using ll = long long;
constexpr ll MOD = 1000000007;

int w, h;
vector<vector<int>> field;
vector<vector<bool>> checked;

void dfs(int y, int x, int& node, int& edge, const int number)
{
    checked[y][x] = true;
    node++;
    if (x + 1 < w and field[y][x + 1] == number) {
        edge++;
        if (not checked[y][x + 1]) {
            dfs(y, x + 1, node, edge, number);
        }
    }
    if (y + 1 < h and field[y + 1][x] == number) {
        edge++;
        if (not checked[y + 1][x]) {
            dfs(y + 1, x, node, edge, number);
        }
    }
    if (x - 1 >= 0 and field[y][x - 1] == number and (not checked[y][x - 1])) {
        dfs(y, x - 1, node, edge, number);
    }
    if (y - 1 >= 0 and field[y - 1][x] == number and (not checked[y - 1][x])) {
        dfs(y - 1, x, node, edge, number);
    }
}

int main()
{
    cin >> w >> h;
    field.resize(h);
    checked.resize(h);
    rep(i, h)
    {
        field[i].resize(w);
        checked[i].resize(w);
        rep(j, w)
        {
            cin >> field[i][j];
            checked[i][j] = false;
        }
    }
    rep(i, h)
    {
        rep(j, w)
        {
            if (not checked[i][j]) {
                int node = 0;
                int edge = 0;
                dfs(i, j, node, edge, field[i][j]);
                if (node < edge + 1) {
                    cout << "possible" << endl;
                    return 0;
                }
            }
        }
    }
    cout << "impossible" << endl;
    return 0;
}