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 com.company.codeforces;



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

public class Solution {
    static class Node{
        int val;
        Node parent;
        int rank;

    }
    static  class pair{
        int start=0;
        int end=0;
        pair(int x,int y){
            start=x;
            end=y;
        }
    }
    static Stack<pair> remove=new Stack<>();
    static HashMap<Integer,Node> map=new HashMap<>();
    static  HashSet<Integer> faultyparent=new HashSet<>();



    static class disjointset{


        public void  makeset(int val){
            Node node=new Node();
            node.val=val;
            node.rank=0;
            node.parent=node;
            map.put(val,node);

        }
        public void union(int data1,int data2){
            Node a=map.get(data1);
            Node b=map.get(data2);

            Node parent1=findparent(a);
            Node parent2=findparent(b);
            if (parent1==parent2){
                remove.push(new pair(data1,data2));
                faultyparent.add(parent1.val);
                return;
            }

            if (parent1.rank<=parent2.rank){
                if (parent1.rank==parent2.rank){
                    parent1.rank+=1;
                    parent2.parent=parent1;
                }else {
                    parent1.parent=parent2;
                }
            }else {
                parent2.parent=parent1;
            }

        }

        private Node findparent(Node node) {
            if (node.parent==node){
                return node;
            }
            node.parent=findparent(node.parent);
            return node.parent;
        }


    }





    public static void main(String[] args) {
        disjointset ds=new disjointset();
        Scanner input=new Scanner(System.in);


        int n=input.nextInt();


        for (int i = 1; i <=n ; i++) {
            ds.makeset(i);
        }
        for (int i = 0; i <n-1 ; i++) {
            int x=input.nextInt();
            int y=input.nextInt();
            ds.union(x,y);
        }
        if (remove.size()==0){
            System.out.println(0);
        }else {

            System.out.println(remove.size());
            for (int i = 1; i <=n ; i++) {
                if (ds.findparent(map.get(i))!=ds.findparent(map.get(1))){
                    ds.union(1,i);
                    pair p=remove.pop();
                    System.out.println(p.start+" "+p.end+" "+i+" "+1);
                }

            }
        }

    }
}


