#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>

using namespace std;

int H, W;

vector<vector<bool>> visit;
vector<vector<int>> field;

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void dfs(int h, int w, int n, int ph, int pw){
    int nh, nw;
    visit[h][w] = 1;

    for (int dir=0; dir < 4; dir++){
        nh = h + dx[dir];
        nw = w + dy[dir];

        if (nh < 0 || nh >= H || nw < 0 || nw >= W) continue;
        if (field[nh][nw] != n) continue;
        if (nh == ph && nw == pw) continue;
        if (visit[nh][nw]){
            cout << "possible" << endl;
            exit(0);
        }

        dfs(nh, nw, n, h, w);
    }
}

int main(){

    cin >> H >> W;
    swap(H, W);
    field.resize(H, vector<int>(W));
    visit.resize(H, vector<bool>(W));

    for (int i=0; i < H; i++){
        for (int j=0; j<W; j++) cin >> field[i][j];
    }

    for (int i=0; i<H; i++){
        for (int j=0; j<W; j++){
            if (!visit[i][j]){
                dfs(i, j, field[i][j], -1, -1);
            }
        }
    }

    cout << "impossible" << endl;

    return 0;
}