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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;

public class Solution {
    List<Integer>[] g;
    int[] used;
    List<String> bad;

    void dfs(int v){
        used[v] = 1;
        for(int i : g[v]){
            if(used[i] == 0){
                dfs(i);
            }else if(used[i] == 1){
                bad.add((v + 1) + " " + (i + 1));
            }
        }
        used[v] = 2;
    }

    void solve() {
        int n = readInt();
        g = new List[n];
        for(int i = 0;i<n;i++){
            g[i] = new ArrayList<>();
        }

        for(int i = 0;i<n - 1;i++){
            int from = readInt() - 1;
            int to = readInt() - 1;
            g[from].add(to);
            g[to].add(from);
        }

        used = new int[n];
        bad = new ArrayList<>();

        List<Integer> comps = new ArrayList<>();
        for(int i = 0;i<n;i++){
            if(used[i] == 0){
                dfs(i);
                comps.add(i + 1);
            }
        }
        List<String> ans = new ArrayList<>();
        while(comps.size() > 1){
            String a = bad.get(bad.size() - 1);
            bad.remove(bad.size() - 1);
            String q = comps.get(comps.size() - 2) + " " + comps.get(comps.size() - 1);
            comps.remove(comps.size() - 1);
            ans.add(a + " " + q);
        }
        out.println(ans.size());
        for(String i : ans){
            out.println(i);
        }
    }

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

    private void run() {
        try {
            init();
            solve();
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }

    private BufferedReader in;
    private StringTokenizer tok = new StringTokenizer("");

    private PrintWriter out;

    private void init() {
        in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);

//        try {
//            in = new BufferedReader(new FileReader("absum.in"));
//            out = new PrintWriter(new File("absum.out"));
//        } catch (FileNotFoundException e) {
//            e.printStackTrace();
//        }
    }

    private String readLine() {
        try {
            return in.readLine();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    private String readString() {
        while (!tok.hasMoreTokens()) {
            String nextLine = readLine();
            if (nextLine == null) return null;
            tok = new StringTokenizer(nextLine);
        }

        return tok.nextToken();
    }

    private int readInt() {
        return Integer.parseInt(readString());
    }

    private long readLong() {
        return Long.parseLong(readString());
    }
}