/// Template path: /home/mohammed/.config/sublime-text-3/Packages/User

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

// typedef
typedef long long ll;
typedef long double ld;
typedef vector<int> vecint;
typedef vector<char> vecchar;
typedef vector<string> vecstr;
typedef vector<long long> vecll;

// Marcos
#define endl ("\n")
#define int long long
#define mod 1000000007
#define pi (3.141592653589)
#define REP(i, a, b) for (int i = a; i < b; i++)
#define RREP(i, a, b) for (int i = a; i > b; i--)
#define fast ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr)

// Functions
long long squared(long long x) { return (x * x) % mod; }
int factorial(int n)
{
    long long res = 1;
    for (int i = 1; i <= n; i++)
    {
        res = ((res * i) % mod + mod) % mod;
    }
    return res;
}
long long power(long long x, long long p)
{
    if (p == 0)
    {
        return 1;
    }
    if (p % 2 == 1)
    {
        return (power(x, p - 1) * x) % mod;
    }
    return squared(power(x, p / 2));
}

// cout << fixed;
// cout << setprecision(4);

// ---------(^_^)--------- //

// vector<vector<int>> check_new(1000, vector<int>(1000, 0));
map<int, map<int, int>> check_new;
vector<vector<int>> edg;

vector<int> vis;
bool e = 0;
vector<vector<int>> to_del;
vector<vector<int>> result1;
vector<vector<int>> result2;
void dfs(int idx, int parent)
{
    if (e)
        return;
    for (auto &i : edg[idx])
    {
        if (vis[i] && i != parent && check_new[i][idx] != -2 && !e)
        {
            e = 1;
            check_new[to_del[0][0]][to_del[0][1]] = -2;
            check_new[to_del[0][1]][to_del[0][0]] = -2;
            result1.push_back({to_del[0][0], to_del[0][1]});
            return;
        }
        else if (vis[i] || check_new[i][idx] == -2)
        {
            continue;
        }
        else
        {
            vis[i] = 1;
            if (check_new[idx][i] == -1)
            {
                to_del.push_back({idx, i});
            }
            dfs(i, idx);
            if (check_new[idx][i] == -1 && !e)
            {
                to_del.pop_back();
            }
            vis[i] = 0;
        }
    }
}

void main_solve()
{

    int n;
    cin >> n;

    edg.resize(n);
    vis.resize(n);
    for (int i = 0; i < n - 1; i++)
    {
        int a, b;
        cin >> a >> b;
        check_new[a][b] = -1;
        check_new[b][a] = -1;
        edg[a].push_back(b);
        edg[b].push_back(a);
    }
    vector<vector<int>> nw;
    for (int i = 0; i < n - 1; i++)
    {
        int a, b;
        cin >> a >> b;
        if (check_new[a][b] == -1)
        {
            check_new[a][b] = 1;
            check_new[b][a] = 1;
            continue;
        }
        nw.push_back({a, b});
    }
    for (int i = 0; i < nw.size(); i++)
    {

        int a = nw[i][0];
        int b = nw[i][1];
        // cout << check_new[4][7] << endl;
        // cout << a << ' ' << b << "  " << check_new[a][b] << endl;

        e = 0;
        vis.clear();
        to_del.clear();
        vis.resize(n);
        check_new[a][b] = 1;
        check_new[b][a] = 1;
        edg[a].push_back(b);
        edg[b].push_back(a);
        vis[a] = 1;
        dfs(a, a);
        result2.push_back({a, b});
    }

    int o = result1.size();
    cout << o << endl;
    for (int i = 0; i < o; i++)
    {
        cout << result1[i][0] << " ";
        cout << result1[i][1] << " ";
        cout << result2[i][0] << " ";
        cout << result2[i][1] << " ";
        cout << endl;
    }
}

int32_t main()
{

    fast;
    // #ifndef ONLINE_JUDGE
    // 	freopen("input.txt", "r", stdin);
    // 	freopen("output.txt", "w", stdout);
    // #endif
    // Just another problem (-_-)

    int t;
    t = 1;
    // cin >> t;

    while (t--)
    {
        main_solve();
    }
}