#include "bits/stdc++.h"
using namespace std;

typedef long long Int;
#define REP(i,n) for(int (i)=0;(i)<(int)(n);++(i))

int W, H;
int a[110][110];

vector<int> g[10000];
bool vis[10000];
bool cycle = false;
void dfs(int p, int prev) {
	if (vis[p]) {
		cycle = true;
		return;
	}
	vis[p] = true;
	REP(i, g[p].size()) if (prev != g[p][i]) {
		dfs(g[p][i], p);
	}
}

int main() {
	cin >> W >> H;
	REP(i, H) REP(j, W) cin >> a[i][j];

	int di[] = {0, 1, 0, -1};
	int dj[] = { 1, 0, -1, 0 };
	REP(i, H) REP(j, W) {
		REP(k, 4) {
			int ni = i + di[k];
			int nj = j + dj[k];
			if (0 <= ni && ni < H && 0 <= nj && nj < W && a[i][j] == a[ni][nj]) {
				int p1 = i * W + j;
				int p2 = ni * W + nj;
				g[p1].push_back(p2);
			}
		}
	}
	cycle = false;
	REP(p, H * W) if (!vis[p]) {
		dfs(p, -1);
	}
	cout << (cycle ? "possible " : "impossible ") << endl;
}