#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin>>
#define ll long long
#define ln cout<<'\n'
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.10f\n",a)
#define mem(a) memset(a,0,sizeof(a))
#define all(c) (c).begin(),(c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
#define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++)
ll check(ll n,ll m,ll x,ll y){return x>=0&&x<n&&y>=0&&y<m;}void pr(){ln;}
template<class A,class...B>void pr(const A &a,const B&...b){cout<<a<<(sizeof...(b)?" ":"");pr(b...);}
template<class A>void PR(A a,ll n){rep(i,n)cout<<(i?" ":"")<<a[i];ln;}
const ll MAX=1e9+7,MAXL=1LL<<61,dx[8]={-1,0,1,0,-1,-1,1,1},dy[8]={0,1,0,-1,-1,1,1,-1};
typedef pair<ll,ll> P;
int n,m;
int d[555][555],a[555][555],f;

void dfs(int x,int y,int px,int py,int c) {
  d[x][y]=1;
  rep(i,4) {
    int xx=x+dx[i],yy=y+dy[i];
    if(check(n,m,xx,yy)&&a[xx][yy]==c&&P(xx,yy)!=P(px,py)) {
      if(d[xx][yy]) {
        f=1;
        continue;
      }
      dfs(xx,yy,x,y,c);
    }
  }
}


void Main() {
  cin >> m >> n;
  rep(i,n)rep(j,m) cin >> a[i][j];
  rep(i,n) {
    rep(j,m) {
      if(!d[i][j]) {
        dfs(i,j,-1,-1,a[i][j]);
        if(f) {
          pr("possible");
          return;
        }
      }
    }
  }
  pr("impossible");
}

int main(){ios::sync_with_stdio(0);cin.tie(0);Main();return 0;}