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

public class Main{
static int[] par;
public static void main(String[] args)throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(br.readLine());
    int[][] arr = new int[n-1][2];
    for(int i=0;i<n-1;i++){
        String[] line = br.readLine().split(" ");
        int u = Integer.parseInt(line[0]);
        int v = Integer.parseInt(line[1]);
        arr[i][0] = u;
        arr[i][1] = v;
    }
    ArrayList<Integer> List = new ArrayList<>();
    par = new int[n+1];
    for(int i=0;i<=n;i++){
        par[i]=i;
    }
    ArrayList<Integer> leader = new ArrayList<>();
    unionFind(arr,List);
    for(int i=1;i<par.length;i++){
        if(par[i]==i){
            leader.add(i);
        }
    }
    StringBuilder sb = new StringBuilder();
    int t = leader.size()-1;
    System.out.println(t);
    for(int i=0;i<t;i++){
        if(i==0){
            sb.append(List.get(i)+" "+List.get(i+1)+" ");
            sb.append(leader.get(i)+" "+leader.get(i+1));
        } else {
            sb.append(List.get(i+1)+" "+List.get(i+2)+" ");
            sb.append(leader.get(i)+" "+leader.get(i+1));
        }
        System.out.println(sb);
        sb = new StringBuilder();
    }
        
 }
 public static void unionFind(int[][] arr, ArrayList<Integer> List){
     int n = arr.length;
     int m = 2;
     for(int[] a : arr){
        int p1 = findPar(a[0]);
        int p2 = findPar(a[1]);
        if(p1 != p2){
            par[p1]=p2;
        } else {
            List.add(a[0]);
            List.add(a[1]);
        }
     }
 }
 public static int findPar(int u){
     if(par[u]==u) return u;
     return par[u] = findPar(par[u]);
 }
}