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

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int W,H; cin>>W>>H;
    vector<vector<int>> grid(W, vector<int>(H));
    for(int i=0;i<H;++i){
        for(int j=0;j<W;++j){
            cin>>grid[i][j];
        }
    }
    vector<vector<int>> g(H*W);
    auto id = [&] (int i, int j){ return i*W+j; };
    for(int i=0;i+1<H;++i){
        for(int j=0;j<W;++j){
            if(grid[i][j]==grid[i+1][j]){
                g[id(i,j)].emplace_back(id(i+1,j));
                g[id(i+1,j)].emplace_back(id(i,j));
            }
        }
    }
    for(int i=0;i<H;++i){
        for(int j=0;j+1<W;++j){
            if(grid[i][j]==grid[i][j+1]){
                g[id(i,j)].emplace_back(id(i,j+1));
                g[id(i,j+1)].emplace_back(id(i,j));
            }
        }
    }

    vector<bool> vis(H*W);
    bool found = false;
    for(int i=0;i<H*W;++i){
        if(vis[i]) continue;
        vis[i]=true;
        int V=1;
        int E=0;
        stack<int> stk; stk.emplace(i);
        while(!stk.empty()){
            auto u = stk.top(); stk.pop();
            E+=g[u].size();
            for(auto& v: g[u]){
                if(vis[v]) continue;
                vis[v]=true;
                V+=1;
                stk.emplace(v);
            }
        }
        E/=2;
        if(V<=E){ found=true; break; }
    }
    cout<<(found?"possible":"impossible")<<endl;
    return 0;
}