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.Arrays;
import java.util.Scanner;

public class Dsu {
	int[] p, rank, setSize;
	int numSets;

	public Dsu(int N) 
	{
		
		numSets = N;
		p = new int[N];
		rank = new int[N];
		setSize = new int[N];
		Arrays.fill(setSize, 1);
		for (int i = 0; i < N; i++) 
			p[i] = i; 
	
	}

	public int findSet(int v) { 
		if (v == p[v])
        return v;
		
    return findSet(p[v]);
	}

	public boolean isSameSet(int i, int j) { return findSet(i) == findSet(j); }

	public void unionSet(int i, int j) 
	{ 
		if (isSameSet(i, j)) 
			return;
		numSets--; 
		int x = findSet(i), y = findSet(j);
		if(setSize[x] > setSize[y]) { 
		p[y] = x;
		setSize[x] += setSize[y]; 
		}
		else
		{	p[x] = y;
		setSize[y] += setSize[x];
		} 
	}

	public int numDisjointSets() { return numSets; }

	public int sizeOfSet(int i) { return setSize[findSet(i)]; }
public static void main(String[] args) {
	Scanner sc=new Scanner(System.in);
	int n=sc.nextInt();
	Dsu d=new Dsu(n);
	ArrayList<Integer> l1=new ArrayList();
	ArrayList<Integer> l2=new ArrayList();
	int u1[]=new int[n];
	int u2[]=new int[n];
	for (int i = 0; i <n-1; i++) {
		int u=sc.nextInt()-1;
		u1[i]=u;
		int v=sc.nextInt()-1;
		u2[i]=v;
		if(d.isSameSet(u, v)) {
		l1.add(u);
		l2.add(v);
		}
		d.unionSet(u, v);
	}
	int days=0;
	int cur=0;
	System.out.println(l1.size());
	for (int i = 0; i <n; i++) {
		for(int j=0 ; j<n ;j++) {
		
		if(!d.isSameSet(i, j)) {
		
			d.unionSet(i, j);
			System.out.println((l1.get(cur)+1)+" "+(l2.get(cur)+1)+" "+(i+1)+" "+(j+1));
			cur++;
			
		}
		}
		
		
		
	}
	
	
	
}

}
