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

#define int long long

typedef long long ll;
typedef pair<int,int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define each(it,v) for(__typeof((v).begin()) it=(v).begin();it!=(v).end();it++)
template<class T,class U>void chmin(T &t,U f){if(t>f)t=f;}
template<class T,class U>void chmax(T &t,U f){if(t<f)t=f;}

int W,H;
int A[111][111];
bool used[111][111];

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

bool dfs(int y,int x,int p){
    used[y][x]=true;
    rep(i,4){
        int ny=y+dy[i],nx=x+dx[i];
        if(ny<0||ny>=H||nx<0||nx>=W||A[ny][nx]!=A[y][x]||p==y*W+x)continue;
        if(used[ny][nx])return true;
        if(dfs(ny,nx,y*W+x))return true;
    }
    return false;
}

signed main(){
    cin>>W>>H;
    rep(i,H)rep(j,W)cin>>A[i][j];

    rep(i,H)rep(j,W){
        memset(used,0,sizeof(used));
        if(dfs(i,j,-1)){
            cout<<"possible"<<endl;
            return 0;
        }
    }
    cout<<"impossible"<<endl;
    return 0;
}