#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <map>

#define Maxn 100005
#define MP(x, y) make_pair(x, y)
#define pii pair <int, int>

using namespace std;

int n, m, deg[Maxn]; bool selfLoop[Maxn];
map <pii, bool> app;

struct Edge {
    int next, to;
}
edge[Maxn * 2];
int head[Maxn], edge_num;

void add_edge(int from, int to) {
    edge[++edge_num].next = head[from];
    edge[edge_num].to = to;
    head[from] = edge_num;
}

bool vis[Maxn]; int maxdeg, mindeg;
void dfs(int u) {
    vis[u] = 1;
    maxdeg = max(maxdeg, deg[u]);
    mindeg = min(mindeg, deg[u]);
    for(int i = head[u]; i; i = edge[i].next) {
        int v = edge[i].to;
        if(vis[v]) continue;
        dfs(v);
    }
}

int main() {
    scanf("%d%d", &m, &n);
    int u, v;
    for(int i = 1; i <= m; ++i) {
        scanf("%d%d", &u, &v);
        if(u > v) swap(u, v);
        if(app.find(MP(u, v)) != app.end()) continue;
        if(u == v) {selfLoop[u] = 1; continue;}
        add_edge(u, v); add_edge(v, u);
        ++deg[u]; ++deg[v]; app[MP(u, v)] = 1;
    }
    for(int i = 1; i <= n; ++i) {
        if(deg[i] == 1 && !selfLoop[i]) vis[i] = 1;
    }
    for(int i = 1; i <= n; ++i) if(vis[i]) --deg[i], --deg[edge[head[i]].to];
    int cnt1 = 0, cnt2 = 0, cnt3 = 0;
    for(int i = 1; i <= n; ++i) {
        maxdeg = 0; mindeg = n + 1;
        if(vis[i]) continue;
        if(!deg[i]) {
            if(selfLoop[i]) ++cnt1;
            vis[i] = 1; continue;
        }
        dfs(i);
        if(maxdeg == 2 && mindeg == 2) ++cnt2;
        else if(maxdeg == 2 && mindeg == 1) ++cnt1;
        else if(maxdeg == 1 && mindeg == 1) ++cnt1;
        else if(maxdeg >= 3) ++cnt3;
        // cerr << "dfs log " << i << " " << mindeg << " " << maxdeg << endl;
    }
    // cout << cnt1 << " " << cnt2 << " " << cnt3 << endl;
    if(cnt3 || (cnt2 > 1) || (cnt2 && cnt1)) cout << "impossible" << endl;
    else cout << "possible" << endl;
    return 0;
}