// ﷽
#include <bits/stdc++.h>

using namespace std;

#ifdef SAWALHY
#include "debug.hpp"
#else
#define debug(...) 0
#define debug_itr(...) 0
#define debug_bits(...) 0
#endif

#define ll long long
#define int long long
#define all(v) v.begin(), v.end()

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL), cout.tie(NULL);

  int n, k;
  cin >> n >> k;

  vector<set<int>> adj(k);

  for (int i = 0, a, b; i < n; i++) {
    cin >> a >> b;
    a--, b--;
    adj[a].insert(b);
    adj[b].insert(a);
  }

  auto newadj = adj;

  for (int i = 0; i < k; i++) {
    if (adj[i].size() == 1) {
      newadj[*adj[i].begin()].erase(i);
    }
    if (newadj[i].size() > 2) {
      newadj[i].erase(i);
    }
  }

  for (int i = 0; i < k; i++) {
    if (newadj[i].size() > 2) {
      cout << "impossible\n";
      exit(0);
    }
  }

  cout << "possible\n";
}