#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (int i=(int)from; i<(int)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (int i=(int)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const int INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(int x=pow(10,6)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { int d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }


int main() {
  ll W, H; cin >> W >> H;
  ll M[H][W]; rep(i,H) rep(j,W) cin >> M[i][j];
  bool used[H][W] = {};

  string result = "impossible";
  rep(i,H) rep(j,W) {
    if (used[i][j]) continue;
    used[i][j] = true;
    queue<p_ll> q; q.push(make_pair(i,j));
    while (q.size()) {
      p_ll now = q.front(); q.pop();
      ll nh = now.first, nw = now.second;
      ll usecount = 0;
      if (nh!=0&&M[nh-1][nw]==M[nh][nw]) {
        if (used[nh-1][nw]) usecount++;
        else { used[nh-1][nw] = true; q.push(make_pair(nh-1,nw)); }
      }
      if (nh!=H-1&&M[nh+1][nw]==M[nh][nw]) {
        if (used[nh+1][nw]) usecount++;
        else { used[nh+1][nw] = true; q.push(make_pair(nh+1,nw)); }
      }
      if (nw!=0&&M[nh][nw-1]==M[nh][nw]) {
        if (used[nh][nw-1]) usecount++;
        else { used[nh][nw-1] = true; q.push(make_pair(nh,nw-1)); }
      }
      if (nw!=W-1&&M[nh][nw+1]==M[nh][nw]) {
        if (used[nh][nw+1]) usecount++;
        else { used[nh][nw+1] = true; q.push(make_pair(nh,nw+1)); }
      }
      if (usecount>=2) result = "possible";
      // cout << nh << "," << nw << " ";
    }
    // cout << endl;
  }
  cout << result << endl;
  return 0;
}