#include <bits/stdc++.h>
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define FORR(i,a,b) for (int i=(a);i>=(b);i--)
#define pb push_back
#define pcnt __builtin_popcount
#define show(x) cout<<#x<<" = "<<x<<endl;
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define fi first
#define se second
#define rng(a) a.begin(),a.end()
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define sz(x) (int)(x).size()

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef set<int> si;
typedef pair<ll,ll> pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<pll> vpll;
typedef set<ll> sl;
template<typename T>string join(vector<T>&v)
{stringstream s;FOR(i,0,sz(v))s<<' '<<v[i];return s.str().substr(1);}
ll gcd(ll a,ll b){if(a>b)swap(a,b);for(;a>0;b%=a,swap(a,b));return b;}
int modpow(ll a,ll n,int m){if(a==0)return a;ll p=1;for(;n>0;n/=2,a=a*a%m)if(n&1)p=p*a%m;return (int)p;}

const int iinf = 1e9;
const ll linf = 4e18;
const int mod = 1e9+7;
const double eps = 1e-10;

int w, h, m[100][100];
bool y[100][100];
set<pii> ss;
bool search(pii pi, pii prev){
  int c = m[pi.fi][pi.se], f = pi.fi, s = pi.se;
  int _f, _s;
  pii t;
  if(ss.find(pi) != ss.end())return true;
  ss.insert(pi);
  y[pi.fi][pi.se] = true;
  if(f > 0){
    _f = f-1;
    _s = s;
    t = pii(_f,_s);
    if(m[_f][_s] == c && prev != t)if(search(t, pi)) return true;
  }
  if(f < h-1){
    _f = f+1;
    _s = s;
    t = pii(_f,_s);
    if(m[_f][_s] == c && prev != t)if(search(t, pi)) return true;
  }
  if(s > 0){
    _f = f;
    _s = s-1;
    t = pii(_f,_s);
    if(m[_f][_s] == c && prev != t)if(search(t, pi)) return true;
  }
  if(s < h-1){
    _f = f;
    _s = s+1;
    t = pii(_f,_s);
    if(m[_f][_s] == c && prev != t)if(search(t, pi)) return true;
  }
  return false;
}
main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> w >> h;
  FOR(i, 0, h){
    FOR(j, 0, w){
      cin >> m[i][j];
    }
  }
  FOR(i, 0, h){
    FOR(j, 0, w){
      if(y[i][j])continue;
      ss.clear();
      if(search(pii(i, j), pii(-100, -100))){
        cout << "possible\n";
        return 0;
      }
    }
  }
  cout << "impossible\n";
  return 0;
}