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

#define REP(i,a,b) for(int i=(a),i##_end_=(b);i<i##_end_;++i)
#define INF 0x3f3f3f3f

static const int M=100003;
vector<int>E[M],TE[M];
int n,k,D[M],d,all;
bool Vis[M],cir;
void Find(int x,int f){
	Vis[x]=1;d++;
	for(auto y:TE[x]){
		if(y!=f){
			//cerr<<x<<"->"<<y<<endl;
			if(Vis[y])
				cir=1;
			else Find(y,x);
		}
	}
}
int main(){
	scanf("%d%d",&n,&k);
	REP(i,0,n){
		int a,b;
		scanf("%d%d",&a,&b);
		E[a].push_back(b);
		E[b].push_back(a);
	}
	REP(a,1,k+1){
		sort(E[a].begin(),E[a].end());
		D[a]=unique(E[a].begin(),E[a].end())-E[a].begin();
	}
	REP(a,1,k+1){
		int t=0;
		REP(i,0,D[a]){
			int b=E[a][i];
			if(b==a)continue;
			if(D[a]>1 && D[b]>1)
				TE[a].push_back(b);
		}
		if(((int)TE[a].size())>2){
			puts("impossible");
			return 0;
		}
	}
	REP(a,1,k+1)if(TE[a].size())++all;
	REP(a,1,k+1)if(!Vis[a]){
		d=0;cir=0;
		Find(a,0);
		if(cir && d!=all){
			puts("impossible");
			return 0;
		}
	}
	puts("possible");
	return 0;
}