#include <bits/stdc++.h>

#define all(x) begin(x), end(x)
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define sz(x) (int)(x).size()

#define endl '\n'
#define forn(i, n) for (int i = 0; i < int(n); i++)
#define fi first
#define se second
#define pb push_back
#define vi vector<int>
#define vll vector<long long>
#define vec vector
#define pii pair<int, int>
#define pll pair<long long, long long>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;

int k, n;
string ok = "possible";
string bad = "impossible";

bool cycle = false;
int components = 0;

void dfs(int cur, vector<vector<int>>& G, vector<int>& vis, int p){
    vis[cur] = 1;
    for(auto it : G[cur]){
        if(!vis[it]){
            dfs(it, G, vis, cur);
        }
        else if(vis[it] == 1 && it != p){
            cycle = true;
        }
    }
    vis[cur] = 2;
}

void solve(){
    cin >> n >> k;
    set<int> s[k + 1];
    vector<pii> v(n + 1);
    vector<int> counter(k + 1);
    vector<vector<int>> G(k + 1);
    for(int i =1; i<=n; ++i){
        cin >> v[i].fi >> v[i].se;

        s[v[i].fi].insert(v[i].se);
        s[v[i].se].insert(v[i].fi);

    }

    for(int i = 1; i<=k; ++i){
        if(s[i].size() > 1){
            counter[i] = 1;
        }
    }

    int a;
    for(int i = 1; i<=k; ++i){
        a = 0;
        for(auto it : s[i]){
            if(it == i){
                continue;
            }
            G[i].push_back(it);
            a += counter[it];
        }
        if(a > 2){
            cout << bad << endl;
            return;
        }
    }

    vector<int> vis(k + 1);
    for(int i = 1; i<=k; ++i){
        if(!counter[i]){
            if(!s[i].empty() && *(s[i].begin()) == i){
                ++components;
            }
            continue;
        }
        if(!vis[i]){
            ++components;
            dfs(i, G, vis, -1);
        }
    }

    if(cycle){
        if(components==1){
            cout << ok << endl;
        }
        else{
            cout << bad << endl;
        }
    }
    else{
        cout << ok << endl;
    }

}


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
//    std::random_device rd;
//    std::mt19937 gen(rd());
//    std::uniform_int_distribution<int> distribution(1, 4);

    solve();

    return 0;
}