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 DSU;

import javafx.util.Pair;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

public class RoadsNotOnlyInBerland {
    public static int find_set(int v,int parent[]){
        if(parent[v]==v){
            return v;
        }
        int par=find_set(parent[v],parent);
        parent[v]=par;
        return par;
    }
    public static void union(int u,int v,int parent[],int size[]){
        int par1=find_set(u,parent);
        int par2=find_set(v,parent);
        if(par1!=par2){
            if(size[par1]<size[par2]){
                parent[par1]=par2;
                size[par2]+=size[par1];
            }
            else{
                parent[par2]=par1;
                size[par1]+=size[par2];
            }
        }
        return;
    }
    public static void main(String[] args)throws IOException {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int n=Integer.parseInt(br.readLine());
        int parent[]=new int[n+1];
        int size[]=new int[n+1];
        for(int i=1;i<=n;i++){
            parent[i]=i;
            size[i]=1;
        }
        ArrayList<Pair<Integer,Integer>> toDelete=new ArrayList<>();
        for(int i=1;i<n;i++){
            String line[]=br.readLine().split(" ");
            int x=Integer.parseInt(line[0]);
            int y=Integer.parseInt(line[1]);
            if(find_set(x,parent)==find_set(y,parent)){
                toDelete.add(new Pair<>(x,y));
            }
            union(x,y,parent,size);
        }
        StringBuilder print=new StringBuilder();
        print.append(toDelete.size()+"\n");
        for(int i=0;i<toDelete.size();i++){
            Pair<Integer,Integer> p=toDelete.get(i);
            int x=p.getKey();
            int y=p.getValue();
            int par=find_set(x,parent);
            for(int j=1;j<=n;j++){
                if(parent[j]==j&&par!=j){
                    print.append(x+" "+y+" "+par+" "+j+"\n");
                    union(par,j,parent,size);
                    break;
                }
            }
        }
        System.out.print(print.toString());
    }
}
