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.Scanner;


public class Main {

    static int [] parent;
    static int [] x;
    static int [] y;
    static boolean [] use;
    static int [] k;
    static int [] kk;
    static int node;
    
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        
        int n = scanner.nextInt();
    
        x = new int [n+1];
        y = new int [n+1];
        use = new boolean[n];
        k = new int[n];
        kk = new int[n];
        parent = new int[n+1];
        
        for(int i=0; i<n-1; i++)
        {
            x[i]= scanner.nextInt();
            y[i]= scanner.nextInt();
        }
        
        for(int i=1; i<=n; i++)
            makeSet(i);
        
        node =n;
        
        for(int i=0; i<n-1; i++)
        {
            union(x[i], y[i],  i);
                
        }
        
        int time=0;
        
        if(node==1)
            System.out.println(0);
        else
        {
            System.out.println(node-1);
            for(int i=0; i<use.length; i++)
            {
                if(use[i])
                {
                    int pp = find(k[i]);
                    
                    for(int j=1; j<=n; j++)
                    {
                        if(parent[j]!=pp)
                        {
                            System.out.println(x[i]+" "+y[i]+" "+k[i]+" "+parent[j]);
                            union(x[i], j, 0);
                            break;
                        }
                    }
                    
                    
                }
            }
        }
        
        
    }
    
    
    private static void makeSet(int i)
    {
        parent[i]=i;
    }
    
    private static int find(int i)
    {
        if(i==parent[i])
            return i;
        else return parent[i]= find(parent[i]);
    }
    
    private static void union(int a, int b, int i)
    {
        a=find(a);
        b=find(b);
        
        if(a!=b)
        {
            node--;
            if(a>b)
                parent[b]=a;
            else
                parent[a]=b;
        }
        else
        {
            k[i]=a;
            use[i] = true;
        }
            
    }

}
