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.*;
import java.lang.*;
import java.io.*;
 
public class Solution
{
    static int[] parent;
    static int[] rank;
    
	public static void main (String[] args) throws java.lang.Exception
	{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter out = new PrintWriter(System.out);
		
		StringTokenizer st = new StringTokenizer(br.readLine());
        int n = Integer.parseInt(st.nextToken());
        parent = new int[n+1];
        
       rank = new int[n+1];
        Arrays.fill(rank,1);
        rank[0] = 0;
        int[][] edges = new int[n][2];
        int k =0;
        for(int i=0;i<n-1;i++){
            st = new StringTokenizer(br.readLine());
            int u = Integer.parseInt(st.nextToken());
            int v = Integer.parseInt(st.nextToken());
            if (union(u,v))
             edges[k++] = new int[]{u,v};
        }
	    edges = Arrays.copyOfRange(edges,0,k);
	    //Arrays.stream(edges).forEach(e->System.out.println(Arrays.toString(e)));
	    out.println(edges.length);
	    //System.out.println(Arrays.toString(rank));
	    HashSet<Integer> roots = new HashSet();
	    
	    for(int i=0; i<rank.length;i++)
	    if(rank[i] > 0) roots.add(i);
	    
	    for(int i=0;i<edges.length;i++){
	        int u = edges[i][0];
	        int r = findRoot(u,roots);
	        roots.remove(r);
	        int v = roots.iterator().next();
	        parent[v] = r;
	        roots.remove(v);
	        String s = edges[i][0]+" "+edges[i][1]+" "+r+" "+v;
	        out.println(s);
	        roots.add(r);
	    }
	    out.close();
	}
	
	public static int findRoot(int u, HashSet<Integer> roots){
	    if(roots.contains(u))
	      return u;
	      
	      return findRoot(parent[u],roots);
	}
	
	public static int find(int u){
	    if(parent[u] == u|| parent[u] == 0)
	     return u;
	    return parent[u] = find(parent[u]); 
	}
	
	 public static boolean union(int u, int v){
	     u = find(u);
	     v = find(v);
	     if(u == v) return true;
	     if(rank[u]>=rank[v]){
	         parent[v] = u;
             rank[u] +=  rank[v];
	         rank[v] = 0;
	     }
	     else{
	         parent[u] = v;
	         rank[v]+=rank[u];
	         rank[u] = 0;
	     }
	     return false;
	 }
}