#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

static const double EPS = 1e-8;
static const double PI = 4.0 * atan(1.0);
static const double PI2 = 8.0 * atan(1.0);

#define REP(i,n)	for(int i=0;i<(int)n;++i)
#define ALL(c)		(c).begin(),(c).end()
#define CLEAR(v)	memset(v,0,sizeof(v))
#define MP(a,b)		make_pair((a),(b))
#define ABS(a)		((a)>0?(a):-(a))
#define FOR(i,s,n)	for(int i=s;i<(int)n;++i)

int W, H;
int M[102][102];

bool ved[102][102];
int ar[4][2] = { { 1, 0 }, { 0, -1 }, { -1, 0 }, { 0, 1 } };

bool dfs(int x, int y, int px, int py) {
	if (ved[x][y]) return false;
	ved[x][y] = true;
	REP(i, 4) {
		int ax = x + ar[i][0], ay = y + ar[i][1];
		if (M[ax][ay] == M[x][y]) {
			if (ved[ax][ay] && (ax != px || ay != py)) return true;
			if (dfs(ax, ay, x, y)) return true;
		}
	}
	return false;
}

int main(int argc, char **argv) {
	cin >> W >> H;
	CLEAR(M);
	REP(y, H) REP(x, W) cin >> M[x + 1][y + 1];
	CLEAR(ved);
	REP(x, W + 2) ved[x][0] = ved[x][H + 1] = true;
	REP(y, H + 2) ved[0][y] = ved[W + 1][y] = true;
	bool res = false;
	FOR(x, 1, W + 1) FOR(y, 1, H + 1) res |= dfs(x, y, x, y);
	cout << (res ? "possible" : "impossible") << endl;
	return 0;
}