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

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define subnb true
#define Lnb true
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
const int N = (int)2e5 + 50;
//const int N = 205;

int n, k;
set<int> G[N];
int sp[N];
int vis[N];

int tsum = 0;

void dfs(int v, int p, int rt, int sum) {
    vis[v] = 1;
    sum += sp[v];
    for (int nxt : G[v]) {
        if(nxt == p) continue;
        if(nxt == rt) {
            // found cycle
            cout << ((sum + 1 == tsum) ? "possible" : "impossible") << '\n';
            exit(0);
        }
        dfs(nxt, v, rt, sum + 1);
    }
}


int main() {
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    cin >> k >> n;
    rep(i, 0, k) {
        int u, v; cin >> u >> v; u--, v--;
        if(u == v) sp[u] = 1;
        else {
            G[u].insert(v);
            G[v].insert(u);
        }
    }
    vi leafs;
    rep(i, 0, n) if(sz(G[i]) == 1 && !sp[i]) leafs.push_back(i);
    for(int i : leafs) {
        int to = *G[i].begin();
        if (sz(G[to]) > 1) {
            G[to].erase(i);
        }
        G[i].clear();
    }
    rep(i, 0, n) {
        if(sz(G[i]) > 2) {
            cout << "impossible" << endl;
            return 0;
        }
    }

    rep(i, 0, n) tsum += sz(G[i]);
    tsum /= 2;
    rep(i, 0, n) tsum += sp[i];

    rep(i, 0, n) {
        if(!vis[i]) dfs(i, -1, i, 0);
    }
    cout << "possible" << endl;


}