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

// @EXPECTED_RESULTS@: WRONG_ANSWER, TIME_LIMIT_EXCEEDED

typedef long long ll;
typedef long double ld;
typedef vector<ll> vi;
typedef vector<vi> vvi;

int main() {
    ll h, w;
    cin >> h >> w;
    vvi p(h, vi(w));
    for (ll y = 0; y < h; y++) {
        string s;
        cin >> s;
        for (ll x = 0; x < w; x++) {
            p[y][x] = s[x];
        }
    }
    for (ll y = 0; y < h; y++) {
        for (ll x = 0; x < w; x++) {
            for (ll s = 1; s < max(w, h); s++) {
                ll Y, X;
                ll a = p[y][x], b = p[(Y = (y + s) % h)][x], c = p[y][(X = (x + s) % w)], d = p[Y][X];
                if (a != b && a != c && a != d && b != c && b != d && c != d) {
                    cout << "possible" << endl;
                    return 0;
                }
            }
        }
    }
    cout << "impossible" << endl;
}