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.*;

public class d {
	
	int n;
	int p[];
	List<Integer> g[];
	public void run(){
		Scanner in = new Scanner(System.in);
		n = in.nextInt();
		g = new LinkedList[n];
		for(int i=0;i<n;i++) g[i] = new LinkedList<Integer>();
		
		p = new int[n];
		for(int i=0;i<n;i++) p[i] = i;
		
		for(int i=0;i<n-1;i++){
			int a = in.nextInt()-1;
			int b = in.nextInt()-1;
			g[a].add(b);
			g[b].add(a);
			
			int id1 = find(a);
			int id2 = find(b);
			
			if (id1!=id2) union(id1, id2);
		}
		
		for(int i=0;i<n;i++) p[i] = find(i);
		
		List<int[]> ret = new LinkedList<int[]>();
		
		while(true){
			int id1 = -1;
			int id2 = -1;
			outter:for(int i=0;i<n;i++)
				for(int j=i+1;j<n;j++)
					if (p[i]!=p[j]){
						id1 = p[i];
						id2 = p[j];
						break outter;
					}
			if (id1 == id2) break;
			boolean first = false;
			HashSet<Integer> set = new HashSet<Integer>();
			for(int i=0;i<n;i++) if (id1 == p[i]) set.add(i);
			
			for(int i : set)
				if (g[i].size() > 1) first = true;
			
			
			if (!first){
				set.clear();
				for(int i=0;i<n;i++) if (id2 == p[i]) set.add(i);
			}
			
			int ver = -1;
			for(int i:set)
				if (g[i].size() > 1){
					ver = i;
					break;
				}
			int left = g[ver].remove(0);
			g[left].remove(new Integer(ver));
			
			int vto = -1;
			
			if (first){
				for(int i=0;i<n;i++) if (id2 == p[i]) {
					vto = i;
					break;
				}
			}else{
				for(int i=0;i<n;i++) if (id1 == p[i]){
					vto = i;
					break;
				}
			}
			
			ret.add(new int[]{ver+1, left+1, ver+1, vto+1});
			
			union(id1, id2);
			for(int i=0;i<n;i++) p[i] = find(i);
		}
		System.out.println(ret.size());
		for(int []a : ret)
			System.out.println(a[0] + " " + a[1] + " " + a[2] + " " + a[3]);
	}
	
	private void union(int x, int y){
		p[x] = y;
	}
	private int find(int x){
		if (x!=p[x]) return find(p[x]);
		return x;
	}
	
	public static void main(String args[]){
		new d().run();
	}
}
