#include<bits/stdc++.h>

#define ll long long
#define pp push_back
#define endl '\n'
#define all(x) x.begin(),x.end()
#define ld long double
#define PI acos(-1)
#define sin(a) sin((a)*PI/180)
#define cos(a) cos((a)*PI/180)
#define ones(x) __builtin_popcountll(x)
//#define int ll

using namespace std;

void Drakon() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
#ifdef Clion
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}

unsigned long long inf = 1e10;
const double EPS = 1e-6;
const int MOD = 1000000007, N = 200005, LOG = 25;

ll gcd(ll x, ll y) {
    return y ? gcd(y, x % y) : x;
}

ll lcm(ll a, ll b) {
    return (a * b) / __gcd(a, b);
}

ll mul(const ll &a, const ll &b) {
    return (a % MOD + MOD) * (b % MOD + MOD) % MOD;
}

ll add(const ll &a, const ll &b) {
    return (a + b + 2 * MOD) % MOD;
}

ll pw(ll x, ll y) {
    ll ret = 1;
    while (y > 0) {
        if (y % 2 == 0) {
            x = mul(x, x);
            y = y / 2;
        } else {
            ret = mul(ret, x);
            y = y - 1;
        }
    }
    return ret;
}

void solve() {
    int n, k;
    cin >> n >> k;
    map<int, int>freq;
    map<int, multiset<int>>mp;
    set<int>alive;
    for (int i = 0; i < n; ++i) {
        int a, b;
        cin >> a >> b;
        alive.insert(a);
        alive.insert(b);
        if(a == b){
            freq[a] ++;
            mp[a].insert(b);
        }
        else{
            freq[a] ++;
            freq[b] ++;
            mp[a].insert(b);
            mp[b].insert(a);
        }
    }

    int f = -1, s = -1;

    while (!alive.empty()){
        if(~f){
            bool ok = (f == s);
            alive.erase(f);
            multiset<int>ms = mp[f];
            vector<int>vec;
            for(auto x : ms){
                if(!alive.count(x))continue;
                vec.push_back(x);
            }
            map<int, int>cur;
            vector<int>rem;
            for(auto x : vec){
                if(rem.empty() || rem.back() != x)rem.pp(x);
                cur[x] ++;
                if(cur[x] == freq[x])rem.pop_back(), alive.erase(x);
            }

            if(rem.size() > 1){
                cout << "impossible";
                return;
            }
            else if(rem.size() == 1){
                f = rem.back();
            }
            else{
                f = -1;
                swap(f, s);
            }
            if(ok)break;
        }
        else{
            int z = *alive.begin();
            alive.erase(z);
            multiset<int>ms = mp[z];
            vector<int>vec;
            for(auto x : ms){
                if(!alive.count(x))continue;
                vec.push_back(x);
            }
            map<int, int>cur;
            vector<int>rem;
            for(auto x : vec){
                if(rem.empty() || rem.back() != x)rem.pp(x);
                cur[x] ++;
                if(cur[x] == freq[x])rem.pop_back(), alive.erase(x);
            }

            if(rem.size() > 2){
                cout << "impossible";
                return;
            }
            else if(rem.size() == 2){
                f = rem[0];
                s = rem[1];
            }
            else if(rem.size() == 1){
                f = rem[0];
            }
        }
    }

    if(alive.size()){
        cout << "impossible";
        return;
    }
    cout << "possible";
}

signed main() {
    Drakon();
    int t = 1;
    //cin >> t;
    while (t--) {
        solve();
    }
}