#include<bits/stdc++.h>
using namespace std;

int n, k, chk[100100], chk2[100100];
set<int> s[100100];
vector<int> v[100100];

int dfs(int u) {
	if(chk2[u]) return 0;
	chk2[u] = 1;

	int re = (int)v[u].size() - 2;
	for(auto x : v[u]) re += dfs(x);
	return re;
}

int main()
{
	ios::sync_with_stdio(0); cin.tie(0);
	cin >> k >> n;
	for(int i=1;i<=k;i++) {
		int a, b; cin >> a >> b;
		s[a].insert(b), s[b].insert(a);
	}

	vector<array<int, 2>> p(n);
	for(int i=1;i<=n;i++) p[i-1] = {s[i].size(), i};
	sort(p.rbegin(), p.rend());
	
	int line = 0, cycle = 0;

	for(auto [SSSS, u] : p) if(!chk[u]) {

		for(auto x : s[u]) {
			if(x == u) {

			} else if(s[x].size() == 1) {
				chk[x] = 1;
			} else {
				v[u].push_back(x);
			}
		}

		if(v[u].size() > 2) return cout << "impossible\n", 0;
		if(v[u].empty()) line++;
	}

	// for(int i=1;i<=n;i++) if(!chk[i]) {
	// 	cout << i << " / ";
	// 	for(auto x : v[i]) cout << x << " ";
	// 		cout << "\n";
	// }

	for(int i=1;i<=n;i++) if(!chk[i] && !chk2[i]) {
		// cout << i << " " << dfs(i) << "\n";
		if(dfs(i)) line++;
		else cycle++;
	}

	// cout << "[" << cycle << " " << line << "]\n";

	if(cycle) {
		cout << (cycle+line != 1 ? "impossible" : "possible");
	} else {
		cout << "possible";
	}
}