#include <cstdio>
#include <algorithm>
#include <stack>
#include <queue>
#include <deque>
#include <vector>
#include <string>
#include <string.h>
#include <cstdlib>
#include <ctime>
#include <cmath>
#include <map>
#include <set>
#include <iostream>
#include <sstream>
#include <cctype>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < n; ++i)
#define rrep(i,n) for(int i = 1; i <= n; ++i)
#define drep(i,n) for(int i = n-1; i >= 0; --i)
#define gep(i,g,j) for(int i = g.head[j]; i != -1; i = g.e[i].next)
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcount
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
inline int in() { int x; scanf("%d",&x); return x;}
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
typedef vector<int> vi;

const int MX = 100005, INF = 1000010000;
const ll LINF = 1000000000000000000ll;
const double eps = 1e-10;
const int di[] = {-1,0,1,0}, dj[] = {0,-1,0,1}; //^<v> dj TAKA ^o^

int w, h;
int a[105][105];
bool used[105][105];
string ans;

void dfs(int i, int j, int pi=-1, int pj=-1) {
  if (used[i][j]) {
    ans = "possible";
    return;
  }
  used[i][j] = true;
  rep(v,4) {
    int ni = i+di[v], nj = j+dj[v];
    if (ni == pi && nj == pj) continue;
    if (ni < 0 || nj < 0 || ni >= h || nj >= w) continue;
    if (a[i][j] != a[ni][nj]) continue;
    dfs(ni,nj,i,j);
  }
}

int main(){
  cin >> w >> h;
  rep(i,h)rep(j,w) cin >> a[i][j];
  ans = "impossible";
  rep(i,h)rep(j,w) {
    if (used[i][j]) continue;
    dfs(i,j);
  }
  cout << ans << endl;
  return 0;
}