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.PrintWriter;
import java.util.ArrayList;
import java.util.Queue;
import java.util.Scanner;
import java.util.Stack;
import java.util.TreeSet;

public class CF25D {
	static class Point{
		int i,j;
		Point(int x,int y){
			i=x;j=y;
		}
	}
	static class DSU{
		Stack<Point> a;
		int[] p;
		int[] rank;
		
		
		DSU(int a){
			this.a =new Stack<>(); 
			p=new int[a];
			for(int i=0;i<a;i++) {
				p[i]=i;
			}
			rank=new int[a];
		}
		
		
		int findP(int x) {
			return p[x]==x?x:(p[x]=findP(p[x]));
		}
	
		
		boolean isSameSet(int x,int y) {
			x=findP(x);
			y=findP(y);
			return(x==y); 
		}
		
		
		int union(int x1,int y1) {
			int x=findP(x1);
			int y=findP(y1);
			if(x==y) {
				a.add(new Point(x1, y1));
				return x;
			}
			if(rank[x]>rank[y]) {
				p[y]=x;
				return x;
			}
			else
				p[x]=y;
				if(rank[x]==rank[y])rank[y]++;
				return y;
		}
		
		
		boolean isR() {
			for(int j=1;j<p.length;j++) {
				if(findP(j)!=findP(0))return false;
			}
			return true;
		}
	}
	public static void main(String[] args) {
		Scanner sc= new Scanner(System.in);
		PrintWriter pw =new PrintWriter(System.out);
		
		int n= sc.nextInt();
		DSU ds = new DSU(n);
		for(int i=0;i<n-1;i++) {
			ds.union(sc.nextInt()-1, sc.nextInt()-1);
		}
		TreeSet<Integer> t =new TreeSet<>();
		for(int i=0;i<n;i++) {
			t.add(ds.p[i]);
		}
		System.out.println(t.size()-1);
		String r="";
		while(!ds.isR()) {
			Point p = ds.a.pop();
			int u =t.first();
			t.remove(t.first());
			int v =t.first();
			t.remove(t.first());
			pw.println((p.i+1)+" "+(p.j+1)+" "+(u+1)+" "+(v+1));	
			
			ds.union(u,v );
		}

		pw.flush();
}

}
