#define REP(i,n) for(int i=0; i<(int)(n); i++)

#include <queue>
#include <cstdio>
#include <functional>

inline int getInt(){ int s; scanf("%d", &s); return s; }

#include <set>

using namespace std;

const int _dx[] = {0,1,0,-1};
const int _dy[] = {-1,0,1,0};
#define IN(x,s,g) ((x) >= (s) && (x) < (g))
#define ISIN(x,y,w,h) (IN((x),0,(w)) && IN((y),0,(h)))

int main(){
  const int w = getInt();
  const int h = getInt();
  vector<vector<int> > g(h, vector<int>(w));
  REP(i,h) REP(j,w) g[i][j] = getInt();

  vector<vector<int> > f(h, vector<int>(w));

  function<bool(int, int, int, int)> dfs = [&](int y, int x, int py, int px){
    if(f[y][x]) return true;
    f[y][x] = true;
    REP(i,4){
      const int xx = x + _dx[i];
      const int yy = y + _dy[i];
      if(ISIN(xx, yy, w, h) && (xx != px || yy != py) && g[y][x] == g[yy][xx])
        if(dfs(yy, xx, y, x)) return true;
    }
    return false;
  };

  bool ans = false;
  REP(i,h) REP(j,w) if(!f[i][j])
    ans |= dfs(i, j, -1, -1);

  puts(ans ? "possible" : "impossible");

  return 0;
}