#include<bits/stdc++.h>
using namespace std;
#define mpr make_pair
#define pb push_back
#define fi first
#define se second

const int maxn=111111;
int n, k;
vector<int>v[maxn];
vector<int>g[maxn];

int f[maxn];
int getf(int x){
    return x==f[x]?x:(f[x]=getf(f[x]));
}
void check1(){
    for(int i=1;i<=n;i++){
        if(v[i].size()==1)continue;
        f[i]=i;
        if(g[i].size()!=2)return;
    }
    for(int i=1;i<=n;i++){
        if(v[i].size()==1)continue;        
        f[getf(i)]=getf(g[i][0]);
        f[getf(i)]=getf(g[i][1]);
    }
    set<int>s;
    for(int i=1;i<=n;i++){
        if(v[i].size()==1)continue;
        s.insert(getf(i));
    }
    if(s.size()<=1){
        cout<<"possible";
        exit(0);
    }
}
void check2(){
    for(int i=1;i<=n;i++){
        if(v[i].size()==1)continue;
        f[i]=i;
    }
    for(int i=1;i<=n;i++){
        if(v[i].size()==1)continue;
        int x,y;
        for(int j:g[i]){
            if(j>i)continue;
            x=getf(i);
            y=getf(j);
            if(x==y)return;
            f[x]=y;
        }
    }
    cout<<"possible";
    exit(0);
}
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    cin >> n >> k;
    for(int i = 1; i <= n; i++) {
        int a,b;
        cin >> a >> b;
        if(a!=b)
        v[a].pb(b),v[b].pb(a);
    }    
    n=k;
    for(int i=1;i<=n;i++){
        sort(v[i].begin(),v[i].end());
        v[i].resize(unique(v[i].begin(),v[i].end())-v[i].begin());
    }   
    for(int i=1;i<=n;i++){
        if(v[i].size()!=1){
            for(int j:v[i]){
                if(v[j].size()!=1)g[i].pb(j);
            }
        }
    }
    for(int i=1;i<=n;i++){
        if(v[i].size()!=1&&g[i].size()>2){
            cout<<"impossible\n";
            return 0;
        }
    }
    check1(),check2();
    cout<<"impossible\n";
    return 0;
}