#include <bits/stdc++.h>
typedef long long int ll;
#define endl            '\n'
#define pb             push_back
#define mp              make_pair
#define pll             pair<ll,ll> 
#define fill(x, y)     memset(x, y, sizeof(x))
#define all(x)         (x).begin(), (x).end()
#define debug(x)       { cerr << #x << " = " << x << endl; }
#define IO	       { ios_base::sync_with_stdio(false); cin.tie(0); }
#define read(x)	       freopen(x, "r", stdin)
#define write(x)       freopen(x, "w", stdout)
#define N       100009

using namespace std;


const double inf = 1e18;
const double pi = acos(-1);

vector<pll> a;
vector<ll> v[N];
bool sp[N]={false};

void solve() {
    ll n, m; cin>> n>> m;
    for(ll i=0; i<n; i++){
        ll g, h; cin>> g>> h;
        if(g>h) swap(g,h);
        if(g==h){
            sp[g]=true;
        }
        else{
            a.pb(mp(g,h));
        }
    }
    for(ll i=0; i<a.size(); i++){
        if(i==0 || (i>0 && a[i]!=a[i-1])){
            v[a[i].first].pb(a[i].second);
            v[a[i].second].pb(a[i].first);
        }
    }
    bool ans = true;
    for(ll i=1; i<=m; i++){
        ll g = 0;
        for(ll j=0; j<v[i].size(); j++){
            ll k = v[i][j];
            if(v[k].size()>1 || sp[k]) g++;
        }
        if(g>2) ans = false;
    }
    (ans)? cout<< "possible\n" : cout<< "impossible\n";
}

int main() {
    IO;
    cout << fixed << setprecision(10);
    solve();
}