#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

#define el '\n'
#define FIO ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef complex<ld> pt;
typedef unsigned long long ull;

template<typename T, typename X>
using hashTable = gp_hash_table<T, X>;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

// mt19937_64 for long long
mt19937 rng(std::chrono::system_clock::now().time_since_epoch().count());
const int N = 1e5 + 7;
set<int> st[N];
int nfso[N];

void doWork()
{
    int n, k;
    cin >> n >> k;
    for(int i = 0; i < n; ++i)
    {
        int a, b;
        cin >> a >> b;
        st[a].insert(b);
        st[b].insert(a);
    }
    for(int i = 1; i <= k; ++i)
    {
        int cnt = 0;
        for(auto &it: st[i])
        {
            if(it == i)
                continue;

            bool ok = st[it].count(it);
            if(ok)
                st[it].erase(it);

            if(st[it].size() > 1)
                cnt++;

            if(ok)
                st[it].insert(it);

        }
        if(cnt > 2)
            return cout << "impossible", void();
    }
    cout << "possible";
}

signed main()
{
    FIO
    int T = 1;
//    cin >> T;
    for(int i = 1; i <= T; i++)
        doWork();
}