#include<iostream>
#include<sstream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<numeric>
#include<functional>
#include<complex>

using namespace std;
#define BET(a,b,c) ((a)<=(b)&&(b)<(c))
#define FOR(i,n) for(int i=0,i##_end=(int(n));i<i##_end;i++)
#define SZ(x) (int)(x.size())
#define ALL(x) (x).begin(),(x).end()
#define MP make_pair
#define FOR_EACH(it,v) for(__typeof(v.begin()) it=v.begin(),it_end=v.end() ; it != it_end ; it++)
typedef vector<int> VI;
typedef vector<VI> VVI;

struct DisjointSet {
  vector<int> group ;
  DisjointSet(int n) : group(n, -1) { ; }
  bool unionSet(int x, int y) {
    x = root(x); y = root(y);
    if (x != y) {
      if (group[y] > group[x]) swap(x, y);
      group[x] += group[y]; 
      group[y] = x;
    }
    return x != y;
  }
  bool is_sameSet(int x, int y) { return root(x) == root(y); }
  int root(int x) { return group[x] < 0 ? x : group[x] = root(group[x]); }
  int size(int x) { return -group[root(x)]; }
};

int M[111][111];

int main()
{
    int w,h;
    cin>>w>>h;
    DisjointSet D(w*h);
    FOR(i,h) FOR(j,w) cin>>M[i][j];

    auto toIndex = [w](int row, int col) {
        return row * w + col;
    };

    FOR(i,h) FOR(j,w){
        if(i + 1 < h) {
            if(M[i][j] == M[i+1][j] && !D.unionSet(toIndex(i,j), toIndex(i+1,j))){
                cout<<"possible"<<endl;
                return 0;
            }
        }
        if(j + 1 < w) {
            if(M[i][j] == M[i][j+1] && !D.unionSet(toIndex(i,j), toIndex(i,j+1))){
                cout<<"possible"<<endl;
                return 0;
            }
        }
    }
    cout<<"impossible"<<endl;
    
    return 0;
}