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 
/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
//package begineer;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.LinkedList;

/**
 *
 * @author asmin
 */
public class RoadsNotOnlyInBerland {
    static class City{
        int[] parent;
        int[] rank;
        public City(int n){
            parent=new int[n+1];
            rank=new int[n+1];
            for(int i=1;i<=n;i++){
                parent[i]=i;
            }
        }
        
        public int find(int n){
            if(parent[n]==n)
                return parent[n];
            parent[n]=find(parent[n]);
            return parent[n];
        }
        
        public void merge(int x,int y){
            int parentX=find(x),parentY=find(y);
            if(parentX==parentY)
                return;
            if(rank[parentX]<=rank[parentY]){
                parent[parentX]=parentY;
                if(rank[parentX]==rank[parentY])
                    rank[parentY]++;
            }
            else{
                parent[parentY]=parentX;
            }
        }
    }
    
    public static void main(String[] arg) throws Exception{
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int n=Integer.parseInt(br.readLine());
        int t=n;
        City city=new City(n);
        LinkedList<Integer> closeX=new LinkedList<>();
        LinkedList<Integer> closeY=new LinkedList<>();
        while(t-->1){
            String[] scan=br.readLine().split(" ");
            int u=Integer.parseInt(scan[0]),v=Integer.parseInt(scan[1]);
            if(city.find(u)==city.find(v)){
                closeX.add(u);
                closeY.add(v);
            }
            else{
                city.merge(u,v);
            }
            
        }
        LinkedList<Integer> buildX=new LinkedList<>();
        LinkedList<Integer> buildY=new LinkedList<>();
        
        for(int i=2;i<=n;i++){
            if(city.find(1)!=city.find(i)){
                city.merge(1, i);
                buildX.add(1);
                buildY.add(i);
            }
        }
        System.out.println(closeX.size());
        for (int i = 0; i < closeX.size() && i<buildX.size(); i++) {
            System.out.println(closeX.remove()+" "+closeY.remove()
                    +" "+buildX.remove()+" "+buildY.remove());
        }
    }
}
