#define ll long long
#define ffor(i,a,b) for (int i=(a);i<(b);i++)
#define rfor(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define rep(i,n) for (int i=0;i<(n);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<cmath>
#include<string>
#include<stack>
#include<queue>
#include<vector>
#include<map>
#define SIZE 100001
#define MOD 1000000007
#define INF 100000000
using namespace std;

int w,h;
int field[101][101];
int used[101][101];
bool d[101][101];
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
bool flag = false;

bool bfs(int y, int x, int py, int px, int num){
	d[y][x] = true; used[y][x] = 1;
	rep(i, 4){
		int nowy = y + dy[i], nowx = x + dx[i];
		if(nowy < 0 || h <= nowy || nowx <0 || w <= nowx) continue;
		//戻るの禁止
		if(nowy == py && nowx == px) continue;
		//違う番号なら
		if(field[nowy][nowx] != num) continue;
		if(used[nowy][nowx]) return true;
		bfs(nowy, nowx, y, x, num);
	}
	return false;
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin >> w >> h;
	rep(i,h)rep(j,w) cin >> field[i][j];

	rep(i,h)rep(j,w){
		if(d[i][j]) continue;
		rep(a,101)rep(b,101) used[a][b] = 0;
		if(bfs(i, j, -1, -1, field[i][j])){
			flag = true;
			break;
		}
	}

	cout << ((flag)? "possible":"impossible") << endl;

}