#include<bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10;
int d[N];
vector<int> G[N];
map<int, int> mp[N];
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, k;
    cin >> n >> k;
    
    for(int i = 1, x, y; i <= n; ++i) {
        cin >> x >> y;
        if(x == y) continue;
        if(x > y) swap(x, y);
        if(mp[x].count(y)) continue;
        mp[x][y] = 1;
        G[x].push_back(y);
        G[y].push_back(x);
        d[x]++, d[y]++;
    }
    vector<int> lf;
    for(int i = 1; i <= n; ++i) if(d[i] == 1) lf.push_back(i);
    for(int i :lf) {
        for(int y : G[i]) {
            d[y]--;
        }
    }
    for(int i = 1; i <= n; ++i) if(d[i] > 2) {
        cout << "impossible\n";
        return 0;
    }
    cout << "possible\n";
    return 0;
}