#include <bits/stdc++.h>
using namespace std;
using ll=long long;
void solve(){
    int W,H;cin>>W>>H;
    vector<vector<int>> grid(H);
    for(int i=0;i<H;++i){
        grid[i].resize(W);
        for(int &x:grid[i])cin>>x;
    }

    auto id=[&](int r,int c)->int{ return r*W+c; };
    vector<vector<int>> g(W*H);
    for(int i=0;i+1<H;++i){
        for(int j=0;j<W;++j){
            if(grid[i][j]==grid[i+1][j]){
                int u=id(i,j);
                int v=id(i+1,j);
                g[u].emplace_back(v);
                g[v].emplace_back(u);
            }
        }
    }
    for(int i=0;i<H;++i){
        for(int j=0;j+1<W;++j){
            if(grid[i][j]==grid[i][j+1]){
                int u=id(i,j);
                int v=id(i,j+1);
                g[u].emplace_back(v);
                g[v].emplace_back(u);
            }
        }
    }

    vector<bool> used(H*W);
    for(int i=0;i<H*W;++i){
        if(used[i])continue;
        used[i]=1;
        int E=0,V=1;
        stack<int> stk{{i}};
        while(!stk.empty()){
            auto u=stk.top(); stk.pop();
            E+=g[u].size();
            for(auto&v:g[u]){
                if(used[v])continue;
                used[v]=true;
                V+=1;
                stk.emplace(v);
            }
        }
        E>>=1;
        if(V<=E){
            cout<<"possible"<<endl;
            return;
        }
    }
    cout<<"impossible"<<endl;
}
int main(){
    ios::sync_with_stdio(0);
    cin.tie(nullptr);
    int T=1; //cin>>T;
    while(T--)solve();
    return 0;
}