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

#define Maxn 100005

using namespace std;

int n, m, deg[Maxn], vis[Maxn];

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;
}

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) continue;
        add_edge(u, v); add_edge(v, u);
        ++deg[u]; ++deg[v];
    }
    for(int i = 1; i <= n; ++i) {
        if(deg[i] == 1) {
            int v = edge[head[i]].to;
            vis[i] = vis[v] = 1;
        }
    }
    for(int i = 1; i <= n; ++i) {
        if(vis[i]) --deg[i];
    }
    bool flag = 1;
    for(int i = 1; i <= n; ++i) {
        if(deg[i] > 2) flag = 0;
    }
    if(flag) cout << "possible" << endl;
    else cout << "impossible" << endl;
    return 0;
}