#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=n-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,-1,0,1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

int main(){
    ll w,h;cin >> w >> h;
    vvl g(h,vl(w));
    rep(i,h)rep(j,w)cin >> g[i][j];
    rep(i,h){
        rep(j,w){
            vvl seen(h,vl(w,-1));
            rep(k,4){
                ll ax=i+dx[k],ay=j+dy[k];
                if(ax<0||ay<0||ax>=h||ay>=w)continue;
                if(g[ax][ay]!=g[i][j])continue;
                seen[i][j]=k;
                seen[ax][ay]=k;
                queue<pl> que;que.push({ax,ay});
                while(!que.empty()){
                    pl v=que.front();que.pop();
                    rep(l,4){
                        ll nx=v.fi+dx[l],ny=v.se+dy[l];
                        if(nx<0||ny<0||nx>=h||ny>=w)continue;
                        if(g[nx][ny]!=g[i][j])continue;
                        if(seen[nx][ny]==k)continue;
                        if(seen[nx][ny]==-1){
                            seen[nx][ny]=k;que.push({nx,ny});
                        }
                        else{
                           //cout << i <<" " << j <<endl;
                            cout << "possible" <<endl;return 0;
                        }
                    }
                }
                /*rep(iii,h){
                    rep(jjj,w){
                        cout << seen[iii][jjj] <<" ";
                    }
                    cout << endl;
                }*/
            }
        }
    }
    cout << "impossible" <<endl;
}