# include <bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
const double PI = acos(-1);
template<class T>constexpr T INF() { return ::std::numeric_limits<T>::max(); }
template<class T>constexpr T HINF() { return INF<T>() / 2; }
template <typename T_char>T_char TL(T_char cX) { return tolower(cX); };
template <typename T_char>T_char TU(T_char cX) { return toupper(cX); };
template<typename T>vector<T> make_v(size_t a){ return vector<T>(a); }
template<typename T,typename... Ts>auto make_v(size_t a, Ts... ts){ return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...)); }
template<typename T,typename V>typename enable_if<is_class<T>::value==0>::type fill_v(T &t, const V &v){ t = v; }
template<typename T,typename V>typename enable_if<is_class<T>::value!=0>::type fill_v(T &t, const V &v){ for(auto &e : t) fill_v(e, v); }
const int vy[] = { -1, -1, -1, 0, 1, 1, 1, 0 }, vx[] = { -1, 0, 1, 1, 1, 0, -1, -1 };
const int dx[4] = { 0,1,0,-1 }, dy[4] = { 1,0,-1,0 };
int popcnt(unsigned long long n) { int cnt = 0; for (int i = 0; i < 64; i++)if ((n >> i) & 1)cnt++; return cnt; }
int d_sum(LL n) { int ret = 0; while (n > 0) { ret += n % 10; n /= 10; }return ret; }
int d_cnt(LL n) { int ret = 0; while (n > 0) { ret++; n /= 10; }return ret; }
LL gcd(LL a, LL b) { if (b == 0)return a; return gcd(b, a%b); };
LL lcm(LL a, LL b) { LL g = gcd(a, b); return a / g*b; };
# define ALL(qpqpq)           (qpqpq).begin(),(qpqpq).end()
# define UNIQUE(wpwpw)        sort(ALL((wpwpw)));(wpwpw).erase(unique(ALL((wpwpw))),(wpwpw).end())
# define LOWER(epepe)         transform(ALL((epepe)),(epepe).begin(),TL<char>)
# define UPPER(rprpr)         transform(ALL((rprpr)),(rprpr).begin(),TU<char>)
# define FOR(i,tptpt,ypypy)   for(LL i=(tptpt);i<(ypypy);i++)
# define REP(i,upupu)         FOR(i,0,upupu)
# define INIT                 std::ios::sync_with_stdio(false);std::cin.tie(0)

int w, h;

int fld[111][111];

struct UnionFind {
	UnionFind(size_t size) : v(size, -1) {}
	vector<int> v;

    int root(int x) {
        return (v[x] < 0 ? x : v[x] = root(v[x]));
    }

    bool is_root(int x) {
        return x == root(x);
    }

    bool same(int x, int y) {
        return root(x) == root(y);
    }

    void unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x != y) {
            if (v[x] > v[y]) swap(x, y);
            v[x] += v[y];
            v[y] = x;
        }
    }

    int size(int x) {
    	return -v[root(x)];
	}
};

int main(){
    INIT;
    cin >> w >> h;
    REP(i, h)REP(j, w)cin >> fld[i][j];
    UnionFind UF(w*h);
    REP(i, h)REP(j, w){
        REP(k, 2){
            int nx = j + dx[k], ny = i + dy[k];
            if(nx >= 0 && nx < w && ny >= 0 && ny < h && fld[i][j] == fld[ny][nx]){
                if(UF.same(i*w + j, ny*w + nx)){
                    cout << "possible" << endl;
                    return 0;
                }
                UF.unite(i*w + j, ny*w + nx);
            }
        }
    }
    cout << "impossible" << endl;
}