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

const int M=1e5+10;
typedef pair<int,int> pii;
pii piz[M];
int cnt[M],deg[M];
char loop;
int fath[M];

int Find(int x) {return x == fath[x] ? x : fath[x] = Find(fath[x]);}

int main(){
	int n,k;
	scanf("%d%d",&n,&k);
    set <int> sss;
	for(int i=1;i<=n;++i){
		int a,b;
		scanf("%d%d",&a,&b);
        sss.insert(a); sss.insert(b);
		if(a>b)swap(a,b);
		piz[i]=make_pair(a,b);
	}
    for (int i = 1; i <= k; i ++) fath[i] = i;
	sort(piz+1,piz+n+1);
	n=unique(piz+1,piz+n+1)-piz-1;
	for(int i=1;i<=n;++i){
		auto [a,b]=piz[i];
		if(a==b){
            continue;
        }
		cnt[a]++,cnt[b]++;
	}
	for(int i=1;i<=n;++i){
		auto [a,b]=piz[i];
		if(a==b)continue;
		if(cnt[b]>=2)deg[a]++;
		if(cnt[a]>=2)deg[b]++;
	}
	for(int i=1;i<=k;++i){
		if(deg[i]>2){
			puts("impossible");
			return 0;
		}
	}
    for (int i = 1; i <= n; i ++) {
        auto [a,b] = piz[i];
        if (a == b) continue;
        a = Find(a); b = Find(b);
        if (a == b) {
            loop = 1;
        }
        fath[a] = b;
    }
    if (loop) {
        set <int> s;
        for (int i = 1; i <= k; i ++)
            if (sss.count(i)) s.insert(Find(i));
        if (s.size() != 1) {
            puts("impossible");
			return 0;
        }
    }
	puts("possible");
	return 0;
}