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.io.*;
import java.math.*;
public class Solution{
    public static int n;
    public static HashSet<String> hs1=new HashSet<String>();
    public static HashSet<String> hs2=new HashSet<String>();
    public static LinkedList<Integer>[] al;
    public static int[] visit;
    public static String tostring(int a,int b){
        StringBuilder sb = new StringBuilder("");
        sb.append(a);
        sb.append('.');
        sb.append(b);
        return sb.toString();
    }
    public static int[] toint(String s){
        int id = s.indexOf('.');
        int[] a = new int[2];
        a[0] = Integer.parseInt(s.substring(0,id));
        a[1] = Integer.parseInt(s.substring(id+1,s.length()));
        return a;
    }
    public static void dfs(int no,int p){
        for(int x:al[no]){
            if(visit[x]==0){
                visit[x] = 1;
                dfs(x,no);
            }else{
                hs1.add(tostring(Math.min(no+1,x+1),Math.max(no+1,x+1)));
            }
        }
    }
    public static void main(String[] args)throws IOException{
        BufferedReader br =  new BufferedReader(new InputStreamReader(System.in));
        PrintWriter out = new PrintWriter(System.out);
        StringTokenizer st = new StringTokenizer(br.readLine());
        n = Integer.parseInt(st.nextToken());
        visit = new int[n];
        al = new LinkedList[n];
        for(int i=0;i<n;i++) al[i] = new LinkedList<Integer>();
        for(int i=0;i<n-1;i++){
            st = new StringTokenizer(br.readLine());
            int a = Integer.parseInt(st.nextToken())-1;
            int b = Integer.parseInt(st.nextToken())-1;
            al[a].add(b);
            al[b].add(a);
        }
        for(int i=0;i<n;i++){
            if(visit[i]==0){
                if(i>0){
                    hs2.add(tostring(1,i+1));
                }
                visit[i] = 1;
                dfs(i,-1);
            }
        }
        int l = hs1.size();
        int[][] res1 = new int[l][2];
        l = 0;
        for(String s:hs1){
            res1[l] = toint(s);
            l++;
        }
        int[][] res2 = new int[l][2];
        l = 0;
        for(String s:hs2){
            res2[l] = toint(s);
            l++;
        }
        out.println(l);
        for(int i=0;i<l;i++){
            out.println(res1[i][0]+" "+res1[i][1]+" "+res2[i][0]+" "+res2[i][1]);
        }
        out.flush();
   }
}