#include <iostream>
#include <vector>
#include <stack>

//#define DEBUG
#ifdef DEBUG
#define SHOWSTACK(a) {std::stack<Pos> tmp(a);std::cout << #a << "\t:";for(int i=0; i<static_cast<int>(a.size()); ++i){std::cout << "(" << tmp.top().x << ", " << tmp.top().y << ") ";tmp.pop();}std::cout << "\n";}
#define SHOWM {std::cout << "----\n";for(const auto& tr : m){for(const auto& tp : tr){std::cout << tp.visited << " ";}std::cout << "\n";}}
#else
#define SHOWSTACK(a)
#define SHOWM
#endif

typedef struct{
    int y, x;
    int value;
    bool visited;
    int Fromdirection; //0:↑, 1:→, 2:↓, 3:←, -1:none
}Pos;

int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    std::vector<int> direction({-1, 0, 0, 1, 1, 0, 0, -1});
    int w, h;
    std::cin >> w >> h;
    std::vector<std::vector<Pos>> m(h, std::vector<Pos>(w));
    for(int i=0; i<h; ++i){
        for(int j=0; j<w; ++j){
            int input;
            std::cin >> input;
            m[i][j].y = i;
            m[i][j].x = j;
            m[i][j].value = input;
            m[i][j].visited = false;
            m[i][j].Fromdirection = -1;
        }
    }
    for(auto& r : m){
        for(auto& p : r){
            if(p.visited) continue;
            p.visited = true;
            SHOWM;
            std::stack<Pos> search;
            search.push(p);
            while(!search.empty()){
                SHOWSTACK(search);
                Pos now = search.top();
                search.pop();
                for(int i=0; i<4; ++i){
                    if(i==now.Fromdirection) continue;
                    int sY = now.y+direction[2*i  ];
                    int sX = now.x+direction[2*i+1];
                    if(sY<0 || sY>=h || sX<0 || sX >= w) continue;
                    if(now.value != m[sY][sX].value) continue;
                    if(!m[sY][sX].visited){
                        m[sY][sX].visited = true;
                        SHOWM;
                        m[sY][sX].Fromdirection = (i+2)%4;
                        search.push(m[sY][sX]);
                    }else{
                        std::cout << "possible\n";
                        return 0;
                    }
                }
            }
        }
    }
    std::cout << "impossible\n";
    return 0;
}