void main() {
	enum dx = [1, 0, -1, 0];
	enum dy = [0, 1, 0, -1];
	const W = readInt;
	const H = readInt;
	auto G = new int[][](H, W);
	foreach (i; 0 .. H) foreach (j; 0 .. W) {
		G[i][j] = readInt;
	}
	auto vis = new bool[][](H, W);
	bool ans = false;
	void dfs(int x, int y, int fx, int fy) {
		vis[x][y] = true;
		foreach (i; 0 .. 4) {
			int nx = x + dx[i], ny = y + dy[i];
			if (nx < 0 || ny < 0 || nx >= H || ny >= W) continue;
			if (nx == fx && ny == fy) continue;
			if (G[x][y] != G[nx][ny]) continue;
			if (vis[nx][ny]) {
				ans = true;
				continue;
			}
			dfs(nx, ny, x, y);
		}
	}
	foreach (i; 0 .. H) foreach (j; 0 .. W) {
		if (!vis[i][j]) dfs(i, j, -1, -1);
	}
	writeln(ans ? "possible" : "impossible");
}

import std,core.bitop;
string[]_R;
string readString(){while(_R.empty){_R=readln.chomp.split;}auto ret=_R.front;_R.popFront;return ret;}
int readInt(){return readString.to!int;}
long readLong(){return readString.to!long;}
ulong readULong(){return readString.to!ulong;}
real readReal(){return readString.to!real;}
bool chmin(T)(ref T A,T B){if(A>B){A=B;return true;}else{return false;}}
bool chmax(T)(ref T A,T B){if(A<B){A=B;return true;}else{return false;}}
int lowerBound(T)(T[]A,T x){int L=-1,R=cast(int)A.length;while(R-L>1){int mid=(L+R)/2;(A[mid]<x?L:R)=mid;}return R;}
int upperBound(T)(T[]A,T x){int L=-1,R=cast(int)A.length;while(R-L>1){int mid=(L+R)/2;(A[mid]<=x?L:R)=mid;}return R;}