#line 2 "header.hpp"

//%snippet.set('header')%
//%snippet.fold()%
#ifndef HEADER_H
#define HEADER_H

// template version 2.0
using namespace std;
#include <bits/stdc++.h>

// varibable settings
const long long INF = 1e18;
template <class T> constexpr T inf = numeric_limits<T>::max() / 2.1;

#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (ll i = (ll)(a); i < (ll)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (ll i = (ll)((b)-1); i >= (ll)(a); --i)
#define r_rep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(...) make_tuple(__VA_ARGS__)
#define ub upper_bound
#define lb lower_bound
#define lpos(A, x) (lower_bound(all(A), x) - A.begin())
#define upos(A, x) (upper_bound(all(A), x) - A.begin())
template <class T> inline void chmax(T &a, const T &b) { if ((a) < (b)) (a) = (b); }
template <class T> inline void chmin(T &a, const T &b) { if ((a) > (b)) (a) = (b); }
template <typename X, typename T> auto make_table(X x, T a) { return vector<T>(x, a); }
template <typename X, typename Y, typename Z, typename... Zs> auto make_table(X x, Y y, Z z, Zs... zs) { auto cont = make_table(y, z, zs...); return vector<decltype(cont)>(x, cont); }

#define divceil(a, b) ((a) + (b)-1) / (b)
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x) sort(all(x)); x.erase(unique(all(x)), x.end())
#define slice(l, r) substr(l, r - l)

typedef long long ll;
typedef long double ld;
using vl = vector<ll>;
using vvl = vector<vl>;
using pll = pair<ll, ll>;

template <typename T>
using PQ = priority_queue<T, vector<T>, greater<T>>;
void check_input() { assert(cin.eof() == 0); int tmp; cin >> tmp; assert(cin.eof() == 1); }

#if defined(PCM) || defined(LOCAL)
#else
#define dump(...) ;
#define dump_1d(...) ;
#define dump_2d(...) ;
#define cerrendl ;
#endif

#endif /* HEADER_H */
//%snippet.end()%
#line 2 "solve.cpp"
template<class T=ll> using vec = vector<T>;
struct Fast { Fast() { std::cin.tie(0); ios::sync_with_stdio(false); } } fast;


int solve() {
    ll w,h;cin>>w>>h;

    vvl m(h, vl(w));
    rep(i, h)rep(j, w){
        cin >> m[i][j];
    }

    vvl used(h, vl(w));
    ll dx[]={1, -1, 0, 0};
    ll dy[]={0, 0, 1, -1};
    rep(ii, h)rep(jj, w){
        if (used[ii][jj]) continue;
        stack<tuple<ll, ll, ll, ll>> st;
        st.push(mt(ii, jj, -1, -1));
        while(!st.empty()){
            auto [i, j, li, lj] = st.top(); st.pop(); 
            used[i][j] = 1;
            rep(k, 4) {
                int ni = i + dx[k];
                int nj = j + dy[k];
                if (is_in(ni, 0, h) && is_in(nj, 0, w) && m[i][j] == m[ni][nj]) {
                    if (ni==li && nj==lj) continue;
                    if (used[ni][nj]){
                        // dump(i, j, ni, nj, li, lj);
                        cout << "possible" << endl;
                        return 0;
                    }
                    else{
                        st.push(mt(ni, nj, i, j));
                    }
                }
            }
        }
    }
    cout << "impossible" << endl;

    return 0; 
}


int main(){/*{{{*/
    solve();
    check_input();
    return 0;
}/*}}}*/

// // grid graphを通常のグラフに格納する。
// int h, w;
// cin >> h >> w;
// int n = h * w;  // 頂点数
// vector<vector<ll>> block(h, vector<ll>(w));
// rep(i, h) {
//     string s;
//     cin >> s;
//     rep(j, w) { block[i][j] = (s[j] == '#' ? 1 : 0); }
// }
// 
// Graph g(n);
// 
// auto nid = [&](int i, int j){return (i*w + j);};
// // int u = nid(i, j);
// auto pos = [&](int u) -> pair<int, int> { return {u/w, u%w}; };
// // auto [i,j] = pos(u);
// 
// int di[] = {1, -1, 0, 0};
// int dj[] = {0, 0, 1, -1};
// 
// // 下と右のみ. rep(k, 4)をrep(k, 2)に変更するのも忘れない。
// // int di[] = {1, 0};
// // int dj[] = {0, 1};
// rep(i, h) rep(j, w) {
//     if (block[i][j]) continue;  // blockから出る辺はない
// }