#include<bits/stdc++.h>
using namespace std;
int n,k,a[100010][2],inc[100010],cir,seq,one[100010],vis[100010];
set<int> s[100010];
vector<int> edge[100010];
int dfs(int x,int y){
	int res=0;
	vis[x]=1;
	for (int i=0;i<edge[x].size();i++)
	if (edge[x][i]!=y){
		if (vis[edge[x][i]]) res=1;
		else res|=dfs(edge[x][i],x);
	}
	return res;
}
int main(){
	scanf("%d%d",&n,&k);
	for (int i=0;i<n;i++){
		scanf("%d%d",&a[i][0],&a[i][1]);
		s[a[i][0]].insert(a[i][1]);
		s[a[i][1]].insert(a[i][0]);
	}
	//short
	for (int i=1;i<=k;i++)
	if (s[i].size()==1) one[i]=1;
	for (int i=1;i<=k;i++)
	if (one[i]&&!s[i].empty()){
		int tmp=*s[i].begin();
		if (s[tmp].size()==1) seq=1;
		s[tmp].erase(i);
	}
	//long
	for (int i=1;i<=k;i++)
	if (!one[i]){
		s[i].erase(i);
		if (s[i].size()==0) seq=1;
		else if (s[i].size()>2){
			printf("impossible\n");
			return 0;
		}
		else{
			for (set<int>::iterator it=s[i].begin();it!=s[i].end();++it)
				edge[i].push_back(*it);
		}
	}
	for (int i=1;i<=k;i++)
	if (!vis[i]&&!one[i]){
		if (dfs(i,0)) cir=1;
		else seq=1;
	}
	if (cir==1&&seq==0) printf("possible\n");
	else if (cir==0) printf("possible\n");
	else printf("impossible\n");
}