#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long int ll;
typedef long double ld;
#define FOR(i,l,r) for(ll i=l;i<r;i++)
#define REP(i,n) FOR(i,0,n)
#define RFOR(i,l,r) for(ll i=r-1;i>=l;i--)
#define RREP(i,n) RFOR(i,0,n)
#define ALL(x) x.begin(),x.end()
#define P pair<ll,ll>
#define F first
#define S second
#define BS(A,x) binary_search(ALL(A),x)
#define LB(A,x) (ll)(lower_bound(ALL(A),x)-A.begin())
#define UB(A,x) (ll)(upper_bound(ALL(A),x)-A.begin())
#define COU(A,x) UB(A,x)-LB(A,x)
template<typename T>using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
using mint=modint1000000007;
vector<vector<ll>>X(102,vector<ll>(102,0));bool ans=0;ll p,q;
vector<P>M={P(1,0),P(0,1),P(-1,0),P(0,-1)};
vector<vector<bool>>SEEN(102,vector<bool>(102,0));
void dfs(ll x,ll y,ll j){
  if(ans)return;
  SEEN[x][y]=1;
  REP(i,4)if(j!=i){
    ll xx=x+M[i].F,yy=y+M[i].S;
    if(X[x][y]==X[xx][yy]){
      if(xx==p&&yy==q)ans=1;else if(!SEEN[xx][yy])dfs(xx,yy,(i+2)%4);
    }
  }
  SEEN[x][y]=0;
  return;
}
signed main(){
  ll W,H;cin>>W>>H;REP(i,H)REP(j,W)cin>>X[i+1][j+1];
  FOR(i,1,H+1)FOR(j,1,W+1){p=i;q=j;dfs(i,j,4);if(ans)break;}
  if(ans)cout<<"possible"<<endl;else cout<<"impossible"<<endl;
  return 0;
}