#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];
int dp[101][101];
Struct start;
bool signal;
bool recursion(Struct now,int color){
        if(signal)return true;
        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 signal=true;
        }
        if(dp[now.h][now.w]!=-1){
                if((now.count-dp[now.h][now.w])>3){
                        return signal=true;
                }
                return false;
        }
        dp[now.h][now.w]=now.count;
        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);
        }
        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];
                        dp[i][j]=-1;
                }
        }
        for(int i=0 ; i<H ; i++ ){
                for(int j=0 ; j<W ; j++ ){
                        if(dp[i][j]!=-1)continue;
                        start.h=i;start.w=j;
                        Struct now;
                        now.h=i;now.w=j;now.count=0;
                        signal=false;
                        if(recursion(now,field[i][j])){
                                cout<<"possible"<<endl;
                                return 0;
                        }
                }
        }
        cout<<"impossible"<<endl;
        return 0;
}