#include <bits/stdc++.h>

#define endl '\n'
#define DEBUG_VAR(x) { cerr << "* "#x" = " << x << endl; }
#define DEBUG_FMT(...) { cerr << "* "; fprintf(stderr, __VA_ARGS__); }

using namespace std;
using LL = long long;

const int N = 4e5+5;

int m, n;
int deg[N];

set<int> e[N];
// vector<int> e[N];
bool vis[N];

void topsort()
{
	queue<int> q;

	for(int i = 1; i <= n; i++) {
		if(deg[i] <= 1) {
			q.push(i);
			vis[i] = true;
		}
	}

	while(q.size()) {
		int x = q.front(); q.pop();

		for(int y : e[x]) {
			e[y].erase(x);
			deg[y]--;
			// if(deg[y] <= 1 && !vis[y]) {
			// 	q.push(y);
			// 	vis[y] = true;
			// }
		}
	}
}

bool ins[N];
int cnt = 0, cnt_times = 0;

void dfs(int x, int fa)
{
	if(ins[x]) cnt++;
	if(vis[x]) return;

	vis[x] = true;
	ins[x] = true;
	for(int y : e[x]) {
		if(fa == y) continue;
		dfs(y, x);
	}
	ins[x] = false;
}

int main()
{
	// freopen("1.in", "r", stdin);
	ios::sync_with_stdio(false); cin.tie(nullptr);

	cin >> m >> n;
	for(int i = 1; i <= m; i++) {
		int x, y;
		cin >> x >> y;
		if(x == y) {
			e[x].insert(x+n);
			e[x+n].insert(x);
		} else {
			e[x].insert(y); e[y].insert(x);
		}	
	}

	n = 2*n;
	for(int x = 1; x <= n; x++) {
		// for(int y : e[x]) {
		// 	// e[x].push_back(y);
		// 	deg[x]++;
		// }
		deg[x] = e[x].size();
	}

	topsort();

	int cur = 0;
	for(int i = 1; i <= n; i++) {
		if(!vis[i]) {
			cur = i;
			break;
		}
	}
	if(!cur) {
		cout << "possible" << endl;
		return 0;
	}

	for(int i = 1; i <= n; i++) {
		if(vis[i]) continue;
		if(deg[i] > 2) {
			cout << "impossible" << endl;
			return 0;
		}
	}

	for(int i = 1; i <= n; i++) {
		if(!vis[i]) {
			dfs(i, -1);
			cnt_times++;
		}
	}

	if(cnt == 0 || (cnt_times == cnt && cnt == 1)) {
		cout << "possible" << endl;
		return 0;
	} else {
		cout << "impossible" << endl;
		return 0;
	}
	return 0;
}