#include<bits/stdc++.h>
using namespace std;
using ll = long long;
struct unionfind{
    int n;
    vector<int> parent;
    vector<int> num;
    
    unionfind(int n):n(n),parent(vector<int>(n)),num(vector<int>(n,1)){
        for(int i = 0; i < n; i++) parent[i] = i;
    }

    int find(int n){
        if (parent[n] == n) return n;
        parent[n] = find(parent[n]);
        return parent[n];
    }

    int merge(int n,int m){
        n = find(n);
        m = find(m);
        if (n==m) return n;
        if (num[n]>num[m]) return merge(m,n);
        parent[m] = n;
        num[n] += num[m];
        return n;
    }
    
    int getnum(int n){
        return num[find(n)];
    }
};

int main(){
    int w,h;
    cin>>w>>h;
    vector<vector<int> >m(h,vector<int>(w));
    for(int i = 0;i<h;i++) for(int j  = 0;j<w;j++) cin>>m[i][j];
    int dx[] = {1,0};
    int dy[] = {0,1};
    bool fn = false;
    unionfind uf(h*w);
    for(int i = 0;i<h;i++){
        for(int j = 0;j<w;j++){
            int now = i * w + j;
            for(int k = 0;k<2;k++){
                int ni = i + dx[k];
                int nj = j + dy[k];
                if(ni>=h||nj>=w) continue;
                if(m[i][j]!=m[ni][nj]) continue;
                int nn = ni * w + nj;
                if(uf.find(now)==uf.find(nn)) fn = true;
                uf.merge(now,nn);
            }
        }
    }
    if(fn) cout<<"possible\n";
    else cout<<"impossible\n";

}