#include "bits/stdc++.h"
#include <gmp.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> veci;
typedef vector<string> vecs;
template<class T,class U> using Hash=unordered_map<T,U>;

#define REP(i, a, n) for(ll i = a; i < n; i++)
#define RREP(i, a, n) for(ll i = n-1; i >= a; i--)
#define rep(i, n) REP(i, 0, n)
#define rrep(i, n) RREP(i, 0, n)
#define MD 1000000007

template<class T> T read(){T a;cin >> a;return a;}
template<class T> void read(T& a){cin >> a;}
template<class T> void rarr(T a, int n){for(int i = 0; i < n; i++) {cin >> a[i];}}
template<class T> void write(T a){cout << a << endl;}
template<class T> void writes(vector<T> a, const char* c = " "){cout << a[0];for(int i = 1; i < (int)a.size(); i++)cout << c << a[i];cout << endl;;}
void write(double a){cout << fixed << setprecision(12) << a << endl;}
void write(long double a){cout << fixed << setprecision(12) << a << endl;}
template<class T> void warr(T a, int n, const char* c = " "){cout << a[0];for(int i = 1; i < n; i++)cout << c << a[i];cout << endl;}
void split(string s, string delim, veci& result){result.clear();string::size_type pos = 0;while(pos != string::npos){string::size_type p = s.find(delim, pos);if(p == string::npos){result.push_back(atoi(s.substr(pos).data()));break;}else {result.push_back(atoi(s.substr(pos, p - pos).data()));}pos = p + delim.size();}}
void split(string s, string delim, vecs& result){result.clear();string::size_type pos = 0;while(pos != string::npos){string::size_type p = s.find(delim, pos);if(p == string::npos){result.push_back(s.substr(pos));break;}else {result.push_back(s.substr(pos, p - pos));}pos = p + delim.size();}}
ll gcd(ll a, ll b){while(true){ll k = a % b;if(k == 0)return b;a = b;b = k;}}
ll comb(ll n, ll m){ll p=1;m=min(m,n-m);for(ll i=1;i<=m;i++){p*=n-i+1;p/=i;}return p;}

struct Info{
    Info(int x_,int y_,int d_):x(x_),y(y_),d(d_){}
    int x,y;
    int d;
};

int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};

int main(void)
{
    int w,h;
    bool vis[100][100]={0};
    int m[100][100]={0};
    cin>>w>>h;
    rep(i,h)rep(j,w)cin>>m[i][j];
    int find=0;
    rep(i,h)rep(j,w){
    if(vis[i][j])continue;
        queue<Info> q;
        int num=m[i][j];
        q.push(Info(j,i,-1));
        vis[i][j]=true;
        while(!q.empty()){
            Info t=q.front();
            q.pop();
            rep(i,4){
                int cx=t.x+dx[i];
                int cy=t.y+dy[i];
                if(i==t.d||cx<0||cy<0||cx>=w||cy>=h)continue;
                if(m[cy][cx]!=num)continue;
                if(vis[cy][cx]){
                    find=num;
                    goto BREAK;
                }
                vis[cy][cx]=true;
                q.push(Info(cx,cy,(i+2)%4));
            }
        }
    }
    BREAK:
    /*
    rep(i,h){
        rep(j,w)cout<<vis[i][j]<<" ";
        cout<<endl;
    }
    */
    if(find)write("possible");
    else write("impossible");
    return 0;
}