import std.stdio, std.conv, std.string, std.bigint;
import std.math, std.random, std.datetime;
import std.array, std.range, std.algorithm, std.container;
string read(){ static string[] ss; while(!ss.length) ss = readln.chomp.split; string res = ss[0]; ss.popFront; return res; }

class Cell{
	Cell[] cells;
	Cell parent;
	int id;
	int value;
	int isDone;
	this(int id, int value){
		this.id = id;
		this.value = value;
	}
	bool hasCycle(){
		debug writeln("(", this.id, ")");
		this.isDone = 1;
		foreach(c; cells){
			if(c.value != this.value) continue;
			if(this.parent && c.id == this.parent.id) continue;
			if(! c.isDone){
				c.parent = this;
				if(c.hasCycle) return 1;
			}
			else return 1;
		}
		return 0;
	}
}

Cell[] cells;
int w, h;
void main(){
	w = read.to!int;
	h = read.to!int;
	foreach(i; 0 .. w * h) cells ~= new Cell(i, read.to!int);
	foreach(i; 0 .. w * h){
		if(i % w != 0) cells[i].cells ~= cells[i - 1];
		if(i % w != w - 1) cells[i].cells ~= cells[i + 1];
		if(i / w != 0) cells[i].cells ~= cells[i - w];
		if(i / w != h - 1) cells[i].cells ~= cells[i + w];
	}
	
	foreach(cell; cells){
		if(cell.isDone) continue;
		else{
			if(cell.hasCycle) writeln("possible");
			else continue;
			return;
		}
	}
	writeln("impossible");
	return;
	
}