#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,fma,abm,mmx,avx,avx2")
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rrep(i, n) for (int i = (int)(n - 1); i >= 0; i--)
#define all(x) (x).begin(), (x).end()
#define sz(x) int(x.size())
#define yn(joken) cout<<((joken) ? "Yes" : "No")<<"\n"
#define YN(joken) cout<<((joken) ? "YES" : "NO")<<"\n"
using namespace std;
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vs = vector<string>;
using vc = vector<char>;
using vd = vector<double>;
using vld = vector<long double>;
using vvi = vector<vector<int>>;
using vvl = vector<vector<ll>>;
using vvs = vector<vector<string>>;
using vvc = vector<vector<char>>;
using vvd = vector<vector<double>>;
using vvld = vector<vector<long double>>;
using vvvi = vector<vector<vector<int>>>;
using vvvl = vector<vector<vector<ll>>>;
using vvvvi = vector<vector<vector<vector<int>>>>;
using vvvvl = vector<vector<vector<vector<ll>>>>;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
const int INF = 1e9;
const ll LINF = 2e18;
template <class T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T& a, const T& b) {
    if (b < a) {
        a = b;
        return 1;
    }
    return 0;
}
bool ispow2(int i) { return i && (i & -i) == i; }
bool ispow2(ll i) { return i && (i & -i) == i; }
template <class T>
vector<T> make_vec(size_t a) {
    return vector<T>(a);
}
template <class T, class... Ts>
auto make_vec(size_t a, Ts... ts) {
    return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}
template <typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (int i = 0; i < int(v.size()); i++) {
        is >> v[i];
    }
    return is;
}
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < int(v.size()); i++) {
        os << v[i];
        if (i < int(v.size()) - 1) os << ' ';
    }
    return os;
}

static uint32_t RandXor(){
    static uint32_t x=123456789;
    static uint32_t y=362436069;
    static uint32_t z=521288629;
    static uint32_t w=88675123;
    uint32_t t;
 
    t=x^(x<<11);
    x=y; y=z; z=w;
    return w=(w^(w>>19))^(t^(t>>8));
}

static double Rand01(){
    return (RandXor()+0.5)*(1.0/UINT_MAX);
}

// merge(x,y):mergeする,未併合ならtrueが,併合済みならfalseが返ってくる
// leader(x):xの根を返す
// size(x):xの属する集合のサイズを返す
// same(x,y):x,yが同じ集合に属するかどうか
// groups():各集合に含まれる要素を返す

struct dsu{
public:
    dsu() : _n(0) {}
    dsu(int n) : _n(n), parent_or_size(n, -1) {}

    int merge(int a, int b){
        assert(0 <= a && a < _n);
        assert(0 <= b && b < _n);
        int x = leader(a), y = leader(b);
        if (x == y)
            return x;
        if (-parent_or_size[x] < -parent_or_size[y])
            swap(x, y);
        parent_or_size[x] += parent_or_size[y];
        parent_or_size[y] = x;
        return x;
    }

    bool same(int a, int b){
        assert(0 <= a && a < _n);
        assert(0 <= b && b < _n);
        return leader(a) == leader(b);
    }

    int leader(int a){
        assert(0 <= a && a < _n);
        if (parent_or_size[a] < 0)
            return a;
        return parent_or_size[a] = leader(parent_or_size[a]);
    }

    int size(int a){
        assert(0 <= a && a < _n);
        return -parent_or_size[leader(a)];
    }

    vector<vector<int>> groups(){
        vector<int> leader_buf(_n), group_size(_n);
        for (int i = 0; i < _n; i++){
            leader_buf[i] = leader(i);
            group_size[leader_buf[i]]++;
        }
        vector<vector<int>> result(_n);
        for (int i = 0; i < _n; i++){
            result[i].reserve(group_size[i]);
        }
        for (int i = 0; i < _n; i++){
            result[leader_buf[i]].push_back(i);
        }
        result.erase(
            remove_if(result.begin(), result.end(),
                           [&](const vector<int> &v)
                           { return v.empty(); }),
            result.end());
        return result;
    }

private:
    int _n;
    // root node: -1 * component size
    // otherwise: parent
    vector<int> parent_or_size;
};

void solve(){
    int W,H;
    cin>>W>>H;
    vvi F(H,vi(W));
    rep(i,H) cin>>F[i];
    dsu UF(H*W);
    rep(i,H){
        rep(j,W){
            if(j!=W-1){
                if(F[i][j]==F[i][j+1]){
                    if(UF.same(i*W+j,i*W+j+1)){
                        cout<<"possible"<<endl;
                        return;
                    }
                    UF.merge(i*W+j,i*W+j+1);
                }
            }
            if(i!=H-1){
                if(F[i][j]==F[i+1][j]){
                    if(UF.same(i*W+j,(i+1)*W+j)){
                        cout<<"possible"<<endl;
                        return;
                    }
                    UF.merge(i*W+j,(i+1)*W+j);
                }
            }
        }
    }
    cout<<"impossible"<<endl;
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    solve();
}