#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
int n, k, cut[N];
bool vis[N];
set<int> st[N];
inline int Next(int i, int la) {
    // printf("Next: %d %d\n", i, la);
    assert(st[i].size() == 2);
    // printf("   %d %d\n", *st[i].begin(), *++st[i].begin());
    return *st[i].begin() ^ *next(st[i].begin()) ^ la;
}
int main() {
    scanf("%d%d", &n, &k);
    for (int i = 1, x, y; i <= n; ++i) {
        scanf("%d%d", &x, &y);
        if (x == y) cut[x] = 1;
        else {
            st[x].insert(y);
            st[y].insert(x);
        }
    }
    for (int i = 1; i <= k; ++i) if (st[i].size() == 1 && !cut[i]) {
        int j = *st[i].begin();
        st[i].erase(j);
        st[j].erase(i);
        cut[j] = 1;
    }
    int mn = 1e9;
    for (int i = 1; i <= k; ++i) if (st[i].size() || cut[i]) {
        if (st[i].size() >= 3) return puts("impossible"), 0;
        mn = min<int>(mn, st[i].size());
    }
    if (mn == 2) {
        int tot = 0;
        for (int s = 1; s <= k; ++s) if (st[s].size() || cut[s]) {
            if (vis[s]) continue;
            for (int i = s, la = *st[s].begin(), tmp; !vis[i]; tmp = i, i = Next(i, la), la = tmp) {
                // printf("%d\n", i);
                vis[i] = 1;
            }
            ++tot;
        }
        puts(tot == 1 ? "possible" : "impossible");
    } else {
        for (int s = 1; s <= k; ++s) if (st[s].size() || cut[s]) {
            if (vis[s]) continue;
            if (st[s].size() == 2) continue;
            vis[s] = 1;
            if (st[s].size() == 0) continue;
            for (int i = *st[s].begin(), la = s, tmp; ; tmp = i, i = Next(i, la), la = tmp) {
                vis[i] = 1;
                if (st[i].size() == 1) break;
            }
        }
        for (int s = 1; s <= k; ++s) if (st[s].size() == 2 && !vis[s]) {
            return puts("impossible"), 0;
        }
        puts("possible");
    }
    return 0;
}