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.ArrayList;
import java.util.Scanner;

public class JavaApplication49 {

    static private Scanner input = new Scanner(System.in);

public static class DSU{
    ArrayList<Integer> parent ;
    ArrayList<Integer> rank ;
    DSU(int n)
    {
         parent = new ArrayList<>(n);
         rank = new ArrayList<>(n);
         for(int i=0 ;i<n;i++)
         {
             parent.add(i);
             rank.add(0);
         }
    }
    
    void makeSet(int x)
    {
        parent.set(x,x);
        rank.set(x, 0);
    }
    int GetParent(int x)
    {
        if(parent.get(x)==x)
            return x;
        return GetParent(parent.get(x));
    }
    boolean union(int x , int y)
    {
        int p1=GetParent(x);
        int p2=GetParent(y);
        if(p1==p2)
            return false;
        else
        {
            if(rank.get(p1)>=rank.get(p2))
            {
                parent.set(p2, p1);
                rank.set(p1,(rank.get(p1)==rank.get(p2))? rank.get(p1)+1: rank.get(p1)+rank.get(p2));
            }
            else
            {
                parent.set(p1, p2);
                rank.set(p2, rank.get(p2)+rank.get(p1));
            }
        }
        return true;
    }
    void printParent()
    {
        for(int i=0 ;i<parent.size();i++)
            System.out.println(i+ "          "+parent.get(i)+"             "+rank.get(i));
    }
    void getOutOfUnion()
    {
        for(int i=0 ;i<parent.size();i++){
            int p1=GetParent(i);
            if(p1!=0)
            {
                System.out.println(1 + " "+(p1+1));
                union(0, p1);
                return;
            }
        }
    }
}

    public static void main(String[] args) {
        int n=input.nextInt();
        ArrayList<Integer>arr= new ArrayList<>();
        DSU d = new DSU(n);
        for(int i=0 ;i<n-1;i++)
        {
            int x = input.nextInt()-1, y = input.nextInt()-1;
            if(!d.union(x, y))
            {
                arr.add(x);
                arr.add(y);
            }
        }
        //d.printParent();
        System.out.println((int)arr.size()/2);
        for(int i=0 ;i<arr.size()-1;i+=2)
        {
            System.out.print(arr.get(i)+1+"  "+(1+arr.get(i+1))+" ");
            d.getOutOfUnion();
        }
        
    }
}
