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

const int MAXN = 100'005;
set<int> ds[MAXN],grafo[MAXN],tot;
int st,cont;
bool vis[MAXN];

bool dfs(int nodo,int last){
    //cout<<"nodo "<<nodo<<" "<<last<<"\n";
    vis[nodo]=true;
    cont++;
    bool res=false;
    
    for(auto i : grafo[nodo]){
        if(vis[i]==false){
            res=dfs(i,nodo);
        }else{
            if(i==st && last!=st){
                return true;
            }
        }
    }
    return res;
}

void solve(int t){
    int n,k;
    cin>>n>>k;
    
    vector<int> a(n),b(n);
    for(int i=0;i<n;i++){
        cin>>a[i]>>b[i];
        if(a[i]==b[i]){
            b[i]=++k;
        }
        //if(a[i]==b[i])continue;
        ds[a[i]].insert(b[i]);
        ds[b[i]].insert(a[i]);
    }
    
    for(int i=0;i<n;i++){
        if(a[i]==b[i]){
            tot.insert(a[i]);
            continue;
        }
        
        if(ds[a[i]].size()>1 && ds[b[i]].size()>1){
            grafo[b[i]].insert(a[i]);
            grafo[a[i]].insert(b[i]);
        }
    }
    
    for(int i=1;i<=k;i++){
        if(ds[i].size()>1){
            tot.insert(i);
        }
    }
    
    for(int i=1;i<=k;i++){
        if(grafo[i].size()>2){
            cout<<"impossible\n";
            return;
        }
    }
    /*
    for(auto i : tot)cout<<i<<" ";
    cout<<"\n";
    
    for(int i=1;i<=k;i++){
        cout<<i<<" = ";
        for(auto j : grafo[i]){
            cout<<j<<" ";
        }
        cout<<"\n";
    }
    */
    
    for(int i=1;i<=k;i++){
        if(vis[i]==false){
            st=i;
            cont=0;
            bool flag = dfs(i,-1);
            
            //cout<<i<<" "<<cont<<" "<<flag<<"\n";
            if(flag && cont==tot.size()){
                cout<<"possible\n";
                return;
            }
            
            if(flag){
                cout<<"impossible\n";
                return;
            }
        }
    }
    
    cout<<"possible\n";
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int t=1;
    //cin>>t;
    for(int i=1;i<=t;i++)solve(i);
    
    return 0;
}