#include <bits/stdc++.h>
#include <stdio.h>
#include <stdlib.h>

/// Defines
#define all(a) a.begin(),a.end()
#define ios std::ios::sync_with_stdio(false);

/// STL
#define vi vector<int>
#define vvi vector< vector<int> >
#define pb push_back
#define mp make_pair
#define mii map<int,int>
#define pii pair<int,int>

/// Iterator
#define forit(it, s) for(__typeof(s.begin()) it = s.begin(); it != s.end(); it++)

/// Constants
#define ll long long
#define mod 1000000007
#define EPS 1e-7

/// File I/O
#define READ(f) freopen(f, "r", stdin)
#define WRITE(f) freopen(f, "w", stdout)

using namespace std;

struct DisjointSet
{
    /// Declarations
    int* parent;
    int connectedComp;
    bool connCompNodes[1100];

    /// Constructor.
    DisjointSet(int n)
    {
        parent = new int[n];
        connectedComp = n;
        for(int i = 0 ; i < n ; i++)
        {
            parent[i] = i;
            connCompNodes[i] = true;
        }
    }

    int findParent(int node)
    {
        if(parent[node] == node)
            return node;
        return parent[node] = findParent(parent[node]);
    }

    /// Returns false if the 2 nodes are already connected, indicating that if we want to connect them again it would generate a cycle.
    bool mergeNodes(int a, int b)
    {
        int parentA = findParent(a);
        int parentB = findParent(b);
        if(parentA == parentB)
            return false;
        parent[parentB] = parentA; /// A is the grandParent now
        connectedComp--;
        connCompNodes[parentB] = false;
        return true;
    }

    int connectedComponents()
    {
        return connectedComp;
    }
};

struct plan {
    int i, j, u, v;
} plan[5000];

int main()
{
    int n, t = 0;
    cin >> n;

    struct DisjointSet dsu(n);

    for (int i = 0; i < n - 1; i++)
    {
        int x, y;
        cin >> x >> y;

        x--, y--;
        if(!dsu.mergeNodes(x, y))
        {
            plan[t].i = x+1;
            plan[t].j = y+1;
            t++;
        }
    }

    int f = 0, uCount = 0, vCount = 0;
    for (int i = 0; i < n; i++)
    {
        if (dsu.connCompNodes[i] == true)
        {
            if (f != 0)
            {
                plan[vCount].v = i+1;
                vCount++;
            }
            plan[uCount].u = i+1;
            uCount++;
            f = 1;
        }
    }

    cout << t << endl;
    for (int i = 0; i < t; i++)
        cout << plan[i].i << " " << plan[i].j << " " << plan[i].u << " " << plan[i].v << endl;

    return 0;
}