#include <cassert>
#include <chrono>
#include <iostream>
#include <numeric>
#include <random>
#include <vector>
using namespace std;
int8_t A[50][4000];

int main() {
    int h, w;
    cin >> h >> w;
    for(int j = 0; j < h; ++j) {
        for(int i = 0; i < w; ++i) {
            char c;
            cin >> c;
            if(c == 'r') A[i][j] = 1 << 0;
            if(c == 'g') A[i][j] = 1 << 1;
            if(c == 'b') A[i][j] = 1 << 2;
            if(c == 'w') A[i][j] = 1 << 3;
        }
    }

    int g  = gcd(h, w);
    int hg = h / g;

    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    auto rnd = [&](long long mod) {
        unsigned long long r = rng();
        return (r * mod) >> 32;
    };

    time_t start = clock();
    while(true) {
        for(int i = 0; i < 1000000; ++i) {
            int xl = rnd(w);
            int xr = rnd(w);
            if(xr < xl) swap(xl, xr);

            int yl      = rnd(h);
            int y_delta = g * rnd(hg);
            int yh      = yl + (xr - xl) + y_delta;
            if(yh >= h) yh -= h;
            if(yh >= h) yh -= h;
            if(yh >= h) yh -= h;
            // cerr << xl << " " << xr << " " << yl << " " << yh << endl;
            if((A[xl][yl] | A[xl][yh] | A[xr][yl] | A[xr][yh]) == 15) {
                cout << "possible\n";
                return 0;
            }
        }
        if(clock() - start > 1.4 * CLOCKS_PER_SEC) break;
    }
    cout << "impossible\n";
}