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.*;
import java.util.*;
public class Solution
{
    public static void main(String ag[])throws Exception
    {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int i,j,k;
        int N=Integer.parseInt(br.readLine());
        
        DSU arr[]=new DSU[N+1];
        for(i=0;i<=N;i++)
        {
            arr[i]=new DSU(i,1); 
        }
        ArrayList<Pair> close=new ArrayList<>();
        ArrayList<Pair> build=new ArrayList<>();
        for(i=1;i<N;i++)
        {
            String S[]=br.readLine().split(" ");
            int a=Integer.parseInt(S[0]);
            int b=Integer.parseInt(S[1]);
            int X=find(a,arr);
            int Y=find(b,arr);
            if(X!=Y)
            {
                if(arr[X].rank<arr[Y].rank)
                {
                    int temp=X;
                    X=Y;
                    Y=temp;
                }
                arr[Y].parent=X;
                if(arr[Y].rank==arr[X].rank)
                arr[X].rank++;
            }
            else 
            {
                close.add(new Pair(a,b));
            }
        }
        
        for(i=2;i<=N;i++)
        {
            int X=find(1,arr);
            int Y=find(i,arr);
            if(X!=Y)
            {
                if(arr[X].rank<arr[Y].rank)
                {
                    int temp=X;
                    X=Y;
                    Y=temp;
                }
                arr[Y].parent=X;
                if(arr[Y].rank==arr[X].rank)
                arr[X].rank++;
                build.add(new Pair(1,i));
            }
        }
        System.out.println(build.size());
        for(i=0;i<build.size();i++)
        {
            System.out.println(close.get(i).a+" "+close.get(i).b+" "+build.get(i).a+" "+build.get(i).b);
        }
    }
    public static int find(int a,DSU arr[])
    {
        
        if(arr[a].parent==a)
        return a;
        return arr[a].parent=find(arr[a].parent,arr);
    }
}
class DSU
{
    int parent;
    int rank;
    DSU(int p,int r)
    {
        parent=p;
        rank=r;
    }
}
class Pair 
{
    int a,b;
    Pair(int p,int q)
    {
        a=p;
        b=q;
    }
}