4
1 4
3 1
3 4
 5
4 1
4 3
5 3
2 4
 21
7 15
13 1
14 3
4 10
2 3
16 18
17 20
16 20
8 4
3 12
2 17
13 11
16 1
13 2
13 5
8 9
6 14
3 17
16 9
13 8
 3
3 1
3 2
 6
5 2
5 3
1 4
3 1
5 6
 60
17 34
46 22
50 44
46 33
41 21
31 33
48 6
38 19
35 60
2 24
49 29
7 53
34 1
19 55
32 1
31 42
27 28
4 53
6 50
21 34
1 10
12 36
54 8
16 13
51 43
45 51
54 20
13 53
34 33
49 33
51 11
59 34
15 5
59 28
30 39
13 30
58 4
34 14
3 9
19 34
4 18
26 48
56 54
3 43
57 25
3 41
35 3
48 44
19 13
54 1
23 31
59 47
5 1
46 40
6 26
20 25
37 5
17 24
20 52
 3
3 2
1 2
 39
6 13
15 39
10 35
31 28
4 21
12 39
3 7
3 13
6 1
5 14
36 28
12 15
18 38
30 29
19 34
36 16
20 22
8 13
38 32
26 39
21 37
1 7
15 27
12 26
8 3
6 14
29 2
25 23
32 21
5 16
32 25
6 8
13 10
23 30
34 37
29 33
28 14
36 5
 10
5 9
8 5
7 6
7 9
3 9
2 1
7 2
3 6
7 1

1
3 4 1 2  0
 3
13 2 1 7 3 17 1 19 13 8 1 21  0
 0
 2
48 44 1 6 6 26 1 12  0
 7
12 15 1 2 1 7 1 9 12 26 1 11 8 3 1 12 6 8 1 17 28 14 1 20 36 5 1 24  2
3 6 1 4 7 1 1 10 
//package Round_25;

import java.util.*;
import java.io.*;

import static java.lang.Math.*;

public class d {

    class DSUTree {
        int [] parent;
        int [] rank;
        
        DSUTree(int n){
            parent = new int[n];
            rank = new int[n];
        }
        
        void makeSet(int v){
            parent[v] = v;
            rank[v] = 0;
        }
        
        int findSet(int v){
            if (v == parent[v])
                return v;
            return parent[v] = findSet(parent[v]);
        }
        
        void unionSet(int a, int b){
            a = findSet(a);
            b = findSet(b);
            if (a != b){
                if (rank[a] < rank[b]){
                    int s = a;
                    a = b;
                    b = s;
                }
                parent[b] = a;
                if (rank[a] == rank[b])
                    rank[a]++;
            }
        }
        
    }
    
    class Pair{
        int first, second;
        Pair(int a, int b){
            first = a; second = b; 
        }
    }
    
    void solve() throws Exception {
        int n = in.nextInt();
        DSUTree tree = new DSUTree(n);
        ArrayList<Pair> close = new ArrayList<Pair>();
        for (int i = 0; i<n; i++)
            tree.makeSet(i);
        
        for (int i = 0; i<n - 1; i++){
            int u = in.nextInt() - 1;
            int v = in.nextInt() - 1;
            if (tree.findSet(u) == tree.findSet(v)){
                close.add(new Pair(u + 1, v + 1));
            }else {
                tree.unionSet(u, v);
            }
        }
        
        ArrayList<Pair> open = new ArrayList<Pair>();
        for (int i = 1; i<n; i++){
            if (tree.findSet(0) != tree.findSet(i)){
                open.add(new Pair(1, i + 1));
                tree.unionSet(0, i);
            }
        }
        
        out.println(open.size());
        for (int i = 0; i<open.size(); i++){
            out.print(close.get(i).first + " " + close.get(i).second + " ");
            out.println(open.get(i).first + " " + open.get(i).second);
            
        }
    }

    FastScanner in;
    PrintWriter out;

    String input = "";
    String output = "";

    void run() {
        try {
            if (input.length() > 0) {
                in = new FastScanner(new BufferedReader(new FileReader(input)));
            } else
                in = new FastScanner(new BufferedReader(new InputStreamReader(
                        System.in)));
            if (output.length() > 0)
                out = new PrintWriter(new FileWriter(output));
            else
                out = new PrintWriter(System.out);

            solve();

            out.flush();
            out.close();
        } catch (Exception ex) {
            ex.printStackTrace();
            out.flush();
            out.close();
        } finally {
            out.close();
        }
    }

    public static void main(String[] args) {
        new d().run();
    }

    class FastScanner {
        BufferedReader bf;
        StringTokenizer st;

        public FastScanner(BufferedReader bf) {
            this.bf = bf;
        }

        public String next() throws IOException {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(bf.readLine());
            return st.nextToken();
        }

        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        public String nextLine() throws IOException {
            return bf.readLine();
        }

        public long nextLong() throws IOException {
            return Long.parseLong(next());
        }

        public double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }

        public int[] readIntArray(int n) throws IOException {
            int[] array = new int[n];
            for (int i = 0; i < n; i++)
                array[i] = this.nextInt();
            return array;
        }

        public long[] readLongArray(int n) throws IOException {
            long[] array = new long[n];
            for (int i = 0; i < n; i++)
                array[i] = this.nextLong();
            return array;
        }

    }

}
