#include <bits/stdc++.h>

using namespace std;

#define int long long
#define ll long long
#define forn(i, n) for (ll i = 0; i < n; ++i)
#define forr(i, s, n) for (ll i = s; i < n; ++i)
#define fori(i, s, n) for (ll i = s; i > n; --i)
#define fora(i, n) for (auto i: n)
#define vi vector<int>
#define vll vector<ll>
#define prints(n) cout << (n) << " "
#define println(n) cout << (n) << "\n"
#define newl() cout << "\n"

const int MOD = 998244353;
// const int MOD = 1e9 + 7;

void solve() {
    ll n; cin >> n;

    vector<vector<ll>> adjv(n, vector<ll>());
    vector<set<ll>> adjs(n, set<ll>());
    forn(i, 2 * n - 3) {
        ll u, v; cin >> u >> v; u--; v--;

        adjv[u].push_back(v);
        adjv[v].push_back(u);

        adjs[u].insert(v);
        adjs[v].insert(u);
    }

    vector<tuple<ll, ll, ll>> s;
    forn(i, n) {
        ll fn = i;

        forn(j, adjv[fn].size()) {
            ll sn = adjv[fn][j];

            forn(k, adjv[sn].size()) {
                ll tn = adjv[sn][k];

                if (adjs[tn].find(fn) != adjs[tn].end()) {
                    vector<ll> v;
                    v.push_back(fn); v.push_back(sn); v.push_back(tn);
                    sort(begin(v), end(v));

                    s.push_back(make_tuple(v[0], v[1], v[2]));
                }
            }
        }

        if (s.size() >= 2) break;
    }

    auto [fa, fb, fc] = s[0];
    auto [sa, sb, sc] = s[1];

    println(3);
    prints(fa + 1); prints(fb + 1); println(fc + 1);
    prints(sa + 1); prints(sb + 1); println(sc + 1);
}

int32_t main() {
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    int t = 1;
    // cin >> t;
    while (t--) solve();

    return 0;
}