#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];
vector<int> adj[N];
int color[N];
int par[N],cyclenumber;
vector<vector<int>> cycles;
int mp[N];
vector<int> vec[N];
void cycleDetection(int u, int p) {
    if (color[u] == 2) {
        return;
    }
    if (color[u] == 1) {
        vector<int> v;
        cyclenumber++;
        int cur = p;
        v.push_back(cur);
        while (cur != u) {
            cur = par[cur];
            v.push_back(cur);
        }
        cycles.push_back(v);
        return;
    }
    par[u] = p;
    color[u] = 1;
    for (int v: adj[u]) {
        if (v == par[u]) {
            continue;
        }
        cycleDetection(v, u);
    }
    color[u] = 2;
}
struct DSU
{
    vector<int> par, sz;
    DSU(int n)
    {
        par = vector<int>(n + 1), sz = vector<int>(n + 1, 1);
        iota(par.begin(), par.end(), 0);
    }

    int findSet(int a)
    {
        return (a == par[a] ? a : par[a] = findSet(par[a]));
    }

    void unionSet(int a, int b)
    {
        a = findSet(a), b = findSet(b);
        if(a == b)
            return;
        if(sz[b] > sz[a])
            swap(a, b);
        par[b] = a, sz[a] += sz[b];
    }
};
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);
    }
    DSU dodo(k+5);
    int ans =0;
    for(int i = 1; i <= k; ++i)
    {
        int cnt = 0;
        for(auto &it: st[i])
        {
            if ( i > it)
            {
                if ( dodo.findSet(i) == dodo.findSet(it))
                    ans++;
            }
            dodo.unionSet(i,it);
            if(it == i)
                continue;
            if(st[it].size() > 1)
            {
                cnt++;
            }

        }
        if(cnt > 2)
            return cout << "impossible", void();
    }
    set<int> st;
    for (int i = 1; i <= k ; ++i) {
        st.insert(dodo.findSet(i));
    }
    if ( ans ==0   || st.size() == 1)
    cout << "possible";
    else
        return cout << "impossible", void();
}

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