#include <iostream>
#include <queue>
#include <map>
#include <vector>
using namespace std;
struct Struct{
        int h,w,count;
        bool operator<(const Struct &other)const{
                if(h==other.h)
                        return w<other.w;
                else
                        return h<other.h;
        }
};
int H,W;
int direction[5]={-1,0,1,0,-1};
int field[101][101];
Struct start;
bool recursion(Struct now,int color,map<Struct,bool>past){
        if(now.h<0 || now.h>=H)return false;
        if(now.w<0 || now.w>=W)return false;
        if(field[now.h][now.w]!=color)return false;
        if(now.h==start.h && now.w==start.w && now.count>3)return true;
        if(past[now])return false;
        past[now]=true;
        bool response=false;
        for(int i=0 ; i<4 ; i++ ){
                Struct next;
                next.h=now.h+direction[i];
                next.w=now.w+direction[i+1];
                next.count=now.count+1;
                response|=recursion(next,color,past);
        }
        return response;
}
int main(){
        cin >> W >> H;
        for(int i=0 ; i<H ; i++ )
                for(int j=0 ; j<W ; j++ )
                        cin >> field[i][j];
        for(int i=0 ; i<H ; i++ ){
                for(int j=0 ; j<W ; j++ ){
                        map<Struct,bool> past;
                        start.h=i;start.w=j;
                        Struct now;
                        now.h=i;now.w=j;now.count=0;
                        if(recursion(now,field[i][j],past)){
                                cout<<"possible"<<endl;
                                return 0;
                        }
                }
        }
        cout<<"impossible"<<endl;
        return 0;
}