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

const int N = 101010;
int n, m, deg[N], vis[N], par[N], vz[N], ez[N];
vector<int> g[N];

int Find(int a) {
    return par[a] = (par[a] == a ? a : Find(par[a]));
}
void Union(int a, int b) {
    a = Find(a); b = Find(b);
    ez[a]++;
    if (a != b) {
        vz[a] += vz[b];
        ez[a] += ez[b];
        par[b] = a;
    }
}
int main() {
    cin.tie(0); ios_base::sync_with_stdio(0);
    set<pair<int, int>> st;
    vector<pair<int, int>> e;
    cin >> m >> n;
    for (int i = 0; i < m; i++) {
        int u, v; cin >> u >> v;
        if (u != v) {
            if (u > v) swap(u, v);
            st.insert({ u, v });
        }
        e.push_back({ u, v });
    }
    for (auto [x, y] : st) {
        g[x].push_back(y);
        g[y].push_back(x);
        ++deg[x]; ++deg[y];
    }
    vector<int> v;
    for (int i = 1; i <= n; i++) {
        if (deg[i] == 1) {
            v.push_back(i);
        }
    }
    for (int x : v) {
        vis[x] = 1;
        --deg[g[x][0]];
    }
    bool f = true;
    for (int i = 1; i <= n; i++) {
        if (deg[i] > 2) f = false;
    }
    iota(par, par + N, 0);
    fill(vz, vz + N, 1);

    for (auto [x, y] : e) {
        if (!vis[x] && !vis[y]) {
            Union(x, y);
        }
    }
    int cnt = 0, g = 0;
    for (int i = 1; i <= n; i++) if (!vis[i] && Find(i) == i) {
        if (vz[i] >= 3 && ez[i] == vz[i]) {
            g = 1;
        }
        ++cnt;
    }
    if (g == 1 && cnt != 1) f = false;
    
    cout << (f ? "possible" : "impossible");
}