#include <bits/stdc++.h>

using namespace std;
typedef long long int ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef long double ld;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define nl "\n"
#define all(v) v.begin(),v.end()

int n, m, t, a, b, c;

set<pii> s;

int cnt[100005];

vector<int> v[100005];

bool vis[100005];

bool gd;

void dfs(int x, int par) {
	vis[x] = true;
	for (auto it : v[x]) {
		if (it == par) continue;
		if (vis[it]) gd = true;
		else dfs(it, x);
	}
}

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin >> n >> m;
	
	for (int i=0; i<n; i++) {
		cin >> a >> b;
		if (a > b) swap(a,b);
		s.insert(mp(a, b));
	}
	
	for (auto it : s) {
		int x = it.first, y = it.second;
		cnt[x]++, cnt[y]++;
	}
	
	for (auto it : s) {
		int x = it.first, y = it.second;
		if (x == y) continue;
		
		if (cnt[y] > 1 && cnt[x] > 1) {
			v[x].pb(y);
			v[y].pb(x);
		}
	}
	
	for (int i=1; i<=m; i++) {
		if (v[i].size() > 2) {
			cout << "impossible";
			return 0;
		}
	}
	
	int cnt = 0;
	
	for (int i=1; i<=m; i++) {
		if (!vis[i] && v[i].size()) {
			dfs(i, -1);
			cnt++;
		}
	}
	
	if (gd && cnt > 1) cout << "impossible";
	else cout << "possible";
}