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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;


public class Main {
	public static void main(String[] args)throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	 	PrintWriter pw = new PrintWriter(System.out);
	 	int n = Integer.parseInt(br.readLine());
	 	int[] mark = new int[n-1];
	 	int[] a = new int[n-1];
	 	int[] b = new int[n-1];
	 	DisjointUnionSets dus = new DisjointUnionSets(n);
	 	for(int i=0;i<n-1;i++){
	 		StringTokenizer st = new StringTokenizer(br.readLine());
	 		a[i] = Integer.parseInt(st.nextToken());
	 		b[i] = Integer.parseInt(st.nextToken());
	 		if(dus.find(a[i]-1)!=dus.find(b[i]-1)){
	 			dus.union(a[i]-1, b[i]-1);
	 		}
	 		
	 		else{
	 			mark[i]=1;
	 		}
	 	}
	 	
	 	int[] freq = new int[n];
	 	for(int i=0;i<n;i++){
	 		freq[dus.find(i)]++;
	 	}
	 	int count=0;
	 	
	 	int max = Integer.MIN_VALUE;
	 	for(int i=0;i<n;i++){
	 		if(freq[i]>0){
	 			count++;
	 		}
	 		max = Math.max(max, freq[i]);
	 	}
	 	
	 	pw.println(count-1);
	 	int[] a1 = new int[count-1];
	 	int[] b1 = new int[count-1];
	 	
	 	int store=0;
	 	for(int i=0;i<n;i++){
	 		if(max == freq[i]){
	 			store = i;
	 			break;
	 		}
	 	}
	 	
	 	int co=0;
	 	for(int i=0;i<n;i++){
	 		if(i!=store){
	 			if(freq[i]>0){
	 				a1[co] = store+1;
	 				b1[co] = i+1;
	 				co++;
	 				
	 				if(co == count-1)
	 					break;
	 				
	 			}
	 		}
	 	}
	 	co=0;
	 	for(int i=0;i<n-1;i++){
	 		if(mark[i]==1){
	 			pw.println(a[i]+" "+b[i]+" "+a1[co]+" "+b1[co]);
	 			co++;
	 			if(co==count-1)
	 				break;
	 		}
	 	}
	 	
	 	pw.close();
	 	
	}
}


class DisjointUnionSets extends Main {

	int[] rank, parent;
	int n;
	
	public DisjointUnionSets(int n) {
		rank = new int[n];
		parent = new int[n];
		this.n=n;
		makeSet();
	}
	
	void makeSet() {
		
		for(int i=0;i<n;i++){  // Initially, all elements are in their own set.
			parent[i] = i;
		}
	}
	
	int find(int x){ // Finds the representative of the set that x is an element of
		if(parent[x]!=x){ 
			//if x is not the parent of itself 
	        // Then x is not the representative of his set,
	        
			return find(parent[x]);
			// so we recursively call Find on its parent
			// and move i's node directly under the representative of this set
		}
		else{
			return x;
		}
			
		//return parent[x];
	}
	
	void union(int x, int y){ // Unites the set that includes x and the set that includes y
		int xRoot = find(x); // Find the representatives (or the root nodes) for the set that includes x
		int yRoot = find(y); // And do the same for the set that includes y
		if(xRoot == yRoot)  // Elements are in the same set, no need to unite anything.
			return;
		if(rank[xRoot]<rank[yRoot]){ // If x's rank is less than y's rank
			parent[xRoot] = yRoot; // Then move x under y  so that depth of tree remains less
		}
		
		else if(rank[yRoot]<rank[xRoot]){ // Else if y's rank is less than x's rank
			parent[yRoot] = xRoot; // Then move y under x so that depth of tree remains less
		}
		
		else{ // Else if their ranks are the same
			parent[yRoot] = xRoot; // Then move y under x (doesn't matter which one goes where)
			rank[xRoot] = rank[xRoot] + 1;  // And increment the the result tree's rank by 1
		}
	}

}
