#include <bits/stdc++.h>

using namespace std;
#define ll long long
int tt, tc;

const int N = 1e5 + 9, ALPHA = 331;

set<int> adj[N];

void solve() {

    int n, k;
    cin >> n >> k;
    for (int i = 0; i < n; ++i) {
        int u, v;
        cin >> u >> v;
        adj[u].insert(v);
        adj[v].insert(u);
    }
    for (int i = 1; i <= k; ++i) {
        int c = 0;
        for (auto v: adj[i])
            c += adj[v].size() > 1;
        if (c > 2) return void(cout << "impossible" << endl);
    }
    cout << "possible" << endl;
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    tt = 1, tc = 1; //cin >> tt;
    while (tt--) solve(), tc++;
}