#include <iostream>
#include <sstream>
#include <math.h>
#include <vector>
#include <array>
#include <algorithm>
#include <queue>
#include <numeric>
#include <map>
#include <set>
#include <bitset>

using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define SUM(a) accumulate(ALL(a), 0)
#define array2(type, x, y) array<array<type, y>, x>
#define vector2(type) vector<vector<type> >
#define out(...) printf(__VA_ARGS__)

typedef pair<int, int> pos;
int pos::*x = &pos::first;
int pos::*y = &pos::second;

int dxy[] = {0, 1, 0, -1, 0};

/*================================*/
int W, H;
int M[100][100];
int check[100][100];

bool solve() {
    REP(y,H) REP(x,W) {
        if (check[x][y]) continue;
        int target = M[x][y];
        queue<pos> que;
        que.push(pos(x, y));
        while (que.size()) {
            pos p = que.front();
            que.pop();
            int cnt = 0;
            REP(i,4) {
                int nx = p.first + dxy[i];
                int ny = p.second + dxy[i+1];
                if (nx < 0 || W <= nx) continue;
                if (ny < 0 || H <= ny) continue;
                if (M[nx][ny] != M[p.first][p.second]) continue;
                if (!check[nx][ny]) {
                    que.push(pos(nx, ny));
                } else {
                    cnt++;
                }
            }
            if (cnt >= 2) return true;
            check[p.first][p.second] = true;
        }
    }
    return false;
}

signed main()
{
    cin >> W >> H;
    REP(y,H)REP(x,W) cin >> M[x][y];

    cout << (solve() ? "possible" : "impossible") << endl;

    return 0;
}