#include<bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

struct dsu {
    int N;
    vector<int> V;
    dsu(int n) { N = n; V.resize(N); rep(i, N) V[i] = i; }
    int leader(int a) { if (V[a] == a) return a; return V[a] = leader(V[a]); }
    void merge(int a, int b) { V[leader(a)] = leader(b); }
    bool same(int a, int b) { return leader(a) == leader(b); }
};

int W, H;
int M[100][100];

int main() {
    cin >> W >> H;
    rep(y, H) rep(x, W) cin >> M[x][y];
    dsu G(H * W);


    bool possible = false;
    rep(y, H) rep(x, W - 1) if (M[x][y] == M[x + 1][y]) {
        int a = y * W + x, b = a + 1;
        if (G.same(a, b)) possible = true;
        G.merge(a, b);
    }
    rep(y, H - 1) rep(x, W) if (M[x][y] == M[x][y + 1]) {
        int a = y * W + x, b = a + W;
        if (G.same(a, b)) possible = true;
        G.merge(a, b);
    }

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

    return 0;
}