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.awt.Point;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;
public class Roads {
		static Scanner scan= new Scanner(System.in);
		int counterDays=0;
		static void union(int []Parent,int []rank,int x,int y){
			int par_x=find(Parent,x);
			int par_y=find(Parent,y);

		   if(rank[par_x]>rank[par_y]){
			   Parent[par_y]=par_x;
		   }

		   else{
			   Parent[par_x]=par_y;
		   }
            if(rank[par_x]==rank[par_y]){
            	rank[par_y]++;
            }




		}

		static int find(int []Parent,int x){
			if(Parent[x]==x || Parent[x]==-1){
				return x;
			}
			else{
				return find(Parent,Parent[x]);
			}
		}





		public static void main(String[] args) {
			int i;

			int n=scan.nextInt();

			int [] Parent = new int [n+1];
			int [] rank=new int[n+1];
			ArrayList<Point> possible=new ArrayList<Point>();
			int countPossible=0;
			Arrays.fill(Parent,-1);
            Arrays.fill(rank, 1);

			for(i=1;i<=n-1;i++){
				int x=scan.nextInt();
				int y=scan.nextInt();

				if(Parent[x]!=-1 &&((Parent[x]==Parent[y])|| (Parent[x]==y))){
					Point w = new Point();
					w.x=x;
					w.y=y;

					possible.add(w);
					countPossible++;
				}
				else{
					union(Parent,rank,x,y);
				}


			}


			boolean flag=false;
			ArrayList<Integer> p=new ArrayList<Integer>();
			int finalone [][]=new int [n][4];
			int o=0;
         while(!flag){
				for(int l=1;l<=n;l++){
					if(Parent[l]==-1 || Parent[l]==l){
						p.add(l);
						if(p.size()>1 &&possible.size()!=0){

							int r=p.remove(0);
							int j=p.remove(0);
							union(Parent,rank,r,j);
							Point t=possible.remove(0);
							finalone[o][0]=t.x;
							finalone[o][1]=t.y;
							finalone[o][2]=r;
							finalone[o][3]=j;
							o++;

						}
						else if(possible.size()==0 ||(p.size()==1 && l==n)){
							flag=true;
						}


					}




				}


		}

System.out.println(o);
for(int u=0;u<o;u++){
	System.out.println(finalone[u][0]+" "+finalone[u][1]+" "+finalone[u][2]+" "+finalone[u][3]);
}



		}
}


