#include<bits/stdc++.h>
using namespace std;
typedef pair<int,int> pii;
int k , n;
set<pair<int,int> > st;
vector<int> E[100005];
int deg[200005];
int in[200005];
int d2[200005];
bool ok[200005];

vector<int> G[100005];
bool vis[100005];
int tv , te;
void dfs(int u)
{
    vis[u] = 1;
    tv++ ;
    for(auto v : G[u]) {
        if(!vis[v]) {
            dfs(v) ;
        }
        te++;
    }
}
int main()
{
   // freopen("in.txt","r",stdin) ;
    ios::sync_with_stdio(false) ; cin.tie(0) ; cout.tie(0) ;
    cin >> k >> n;
    for(int i = 1;i <= k;i++) {
        int a , b;cin >> a >> b;
        if(a > b) swap(a, b) ;
        if(st.count({a , b})) continue ;
        st.insert({a , b}) ;
        if(a == b) {
            in[a] = 1;
        }
        else {
            deg[a]++ ; deg[b]++ ;
            E[a].push_back(b) ; E[b].push_back(a) ;
        }
    }
    for(int i = 1;i <= n;i++) {
        if(deg[i] == 1 && !in[i]) ok[i] = 0;
        else ok[i] = 1;
    }
    int mx = 0;
    for(int i = 1;i <= n;i++) {
        if(!ok[i]) continue ;
        for(auto v : E[i]) {
            if(ok[v]) {d2[i]++ ;
            G[i].push_back(v) ;}
        }
        if(ok[i]) mx = max(mx , d2[i]) ;
    }
    if(mx <= 2) {
        int has_l = 0 , has_c = 0;
        for(int i = 1;i <= n;i++) {
            if(ok[i] && !vis[i]) {
                tv = 0 , te = 0;
                dfs(i) ; te /= 2;
                if(tv == 1 && !in[i]) continue ;
                if(tv == te) has_c++;
                else has_l++;
            }
        }
        if((has_c && has_l) || (has_c >= 2)) cout << "impossible";
        else cout << "possible" ;
    }
    else cout << "impossible" ;
    return 0;

}