#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

int dy[] = {-1, 0, 1, 0};
int dx[] = {0, 1, 0, -1};

bool dfs(int y, int x, const vector<string>& s, vector<vector<int> >& index)
{
    int h = s.size();
    int w = s[0].size();

    for(int i=0; i<4; ++i){
        int y2 = y + dy[i];
        int x2 = x + dx[i];
        if(y2 < 0 || y2 >= h || x2 < 0 || x2 >= w || s[y][x] != s[y2][x2])
            continue;

        if(index[y2][x2] == -1){
            index[y2][x2] = index[y][x] + 1;
            if(dfs(y2, x2, s, index))
                return true;
        }
        else if(index[y][x] - index[y2][x2] >= 3){
            return true;
        }
    }
    return false;
}

int main()
{
    int w, h;
    cin >> w >> h;
    vector<string> s(h, string(w, ' '));
    for(int i=0; i<h; ++i){
        for(int j=0; j<w; ++j){
            cin >> s[i][j];
        }
    }

    vector<vector<int> > index(h, vector<int>(w, -1));
    for(int y=0; y<h; ++y){
        for(int x=0; x<w; ++x){
            if(index[y][x] != -1)
                continue;

            index[y][x] = 0;
            if(dfs(y, x, s, index)){
                cout << "possible" << endl;
                return 0;
            }
        }
    }

    cout << "impossible" << endl;
    return 0;
}