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

public class Task25d{
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }
}

class Task{
    private class Edge{
        int a, b;
        public Edge(int t, int i){
            a = t;
            b = i;
        }
    }

    private List<Edge>[] g;
    private ArrayList<Edge> e;
    private boolean[] vis, used;
    private ArrayList<Integer> treePar;
    private int[] par;

    private void dfs(int root){
        if (vis[root])return ;
        vis[root] = true;
        for (Edge v : g[root]){
            if (!vis[v.a]){
                par[v.a] = root;
                used[v.b] = true;
            }
            dfs(v.a);
        }
    }

    public void solve(int testNumber, InputReader in, PrintWriter out) {
        int n = in.nextInt();
        g = new List[n+1];
        e = new ArrayList<>();
        vis = new boolean[n+1];
        used = new boolean[n+1];
        par = new int[n+1];
        treePar = new ArrayList<>();

        for (int i = 0; i<n+1; i++)
            g[i] = new ArrayList<>();

        for (int i = 0; i<n-1; i++){
            int u = in.nextInt();
            int v = in.nextInt();
            g[u].add(new Edge(v, i));
            g[v].add(new Edge(u, i));
            e.add(new Edge(u, v));
        }

        for (int i = 1; i<=n; i++){
            if (!vis[i]){
                treePar.add(i);
                dfs(i);
            }
        }
        ArrayList<Integer> notUsed = new ArrayList<>();
        for (int i = 0; i<n-1; i++){
            if (!used[i]){
                notUsed.add(i);
            }
        }
        if (treePar.size()-1 != notUsed.size()){
            throw new RuntimeException();
        }
        out.println(notUsed.size());
        for (int i = 0; i<notUsed.size(); i++){
            out.println(e.get(notUsed.get(i)).a + " " + e.get(notUsed.get(i)).b + " " + treePar.get(i) + " " + treePar.get(i+1));
        }
    }
}

class InputReader {
    public BufferedReader reader;
    public StringTokenizer tokenizer;

    public InputReader(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream), 32768);
        tokenizer = null;
    }

    public String next() {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }
    public long nextLong() {
        return Long.parseLong(next());
    }
    public double nextDouble() {
        return Double.parseDouble(next());
    }
}