#include <bits/stdc++.h>

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

const int N = 2e5 + 5, mod = 1e9 + 7;

set<int> tmpAdj[N];
vector<int> adj[N];

int n, k, vis[N];

bool dfs(int u, int p = -1) {
    
    if (vis[u]) return true;
    vis[u] = 1;
    
    int ret = 0;
    for (auto v: adj[u]) if (v != p) ret |= dfs(v, u);
    return ret;
    
}

void solve() {
    
    cin >> n >> k;
    for (int i = 0; i < n; ++i) {
        int u, v;
        cin >> u >> v;
        tmpAdj[u].insert(v);
        tmpAdj[v].insert(u);
    }
    
    for (int i = 1; i <= k; ++i) if (tmpAdj[i].empty() || tmpAdj[i].size() == 1 && *tmpAdj[i].begin() != i) vis[i] = 1;
    
    for (int u = 1; u <= k; ++u) {
        
        if (vis[u]) continue;
        
        for (auto v: tmpAdj[u])
            if (!vis[v] && v != u)
                adj[u].push_back(v);
        
        if (adj[u].size() > 2) return void(cout << "impossible" << endl);
    }
    
    int cmps = 0, cycle = 0;
    
    for (int i = 1; i <= k; ++i) {
        if (vis[i]) continue;
        cmps++;
        cycle |= dfs(i);
    }
    if ((cmps > 1) + cycle > 1) cout << "impossible" << endl;
    else cout << "possible" << endl;
    
    
}

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