#include <iostream>
#include <deque>
using namespace std;
#define REP(i, n)           for(int(i)=0;(i)<(n);++(i))
#define mp                  make_pair

const int LIM = 100;
const int dir[][2] = {{1,0},{0,1},{-1,0},{0,-1}};

int W,H;
int board[LIM+1][LIM+1];
pair<int,int> parent[LIM+1][LIM+1];
const pair<int,int> NODATA = mp(-1,-1);

bool solve(){
    REP(y,H) REP(x,W) parent[y][x] = NODATA;
    REP(y,H) REP(x,W){
        if(parent[y][x] != NODATA) continue;
        int c = board[y][x];
        parent[y][x] = mp(x,y);
        deque<pair<int,int> > q;
        q.push_back(mp(x,y));
        while(!q.empty()){
            int nx = q.front().first, ny = q.front().second; q.pop_front();
            REP(d,4){
                int mx = nx + dir[d][0];
                int my = ny + dir[d][1];
                if(mx < 0 || my < 0 || mx >= W || my >= H) continue;
                if(board[my][mx] != c || parent[ny][nx] == mp(mx,my)) continue;
                if(parent[my][mx] != NODATA){ return true; }
                parent[my][mx] = mp(nx,ny);
                q.push_back(mp(mx,my));
            }
        }
    }
    return false;
}

int main(){
    cin >> W >> H;
    REP(y,H) REP(x,W) cin >> board[y][x];
    cout << (solve()?"possible":"impossible") << endl;
    return 0;
}