#include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(int)(a); (i)<(int)(b); ++(i))
#define rFor(i, a, b) for(int (i)=(int)(a)-1; (i)>=(int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
template<class T> bool chmax(T &a, const T &b){if(a<b){a=b; return true;} return false;}
template<class T> bool chmin(T &a, const T &b){if(a>b){a=b; return true;} return false;}
template<class T> T div_floor(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>=0 ? a/b : (a+1)/b-1;
}
template<class T> T div_ceil(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>0 ? (a-1)/b+1 : a/b;
}

constexpr lint mod = 1e9+7;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

int h, w, a[110][110], pv[110][110];
int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

int idx(int x, int y){
    return x*w + y;
}

bool dfs(int x, int y, int v){
    if(pv[x][y] >= 0){
        if(pv[x][y] != v){
            return true;
        }
        return false;
    }
    pv[x][y] = v;
    rep(i, 4){
        int nx = x + dx[i], ny = y + dy[i];
        if(0<=nx && nx<h && 0<=ny && ny<w && a[nx][ny] == a[x][y] && idx(nx, ny) != v){
            if(dfs(nx, ny, idx(x, y))) return true;
        }
    }
    return false;
}

int main(){
    scanf("%d%d", &w, &h);
    rep(i, h)rep(j, w){
        scanf("%d", &a[i][j]);
        pv[i][j] = -1;
    }
    rep(i, h)rep(j, w)if(pv[i][j] < 0){
        if(dfs(i, j, -1)){
            puts("possible");
            return 0;
        }
    }
    puts("impossible");
}