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.*;

public class berland
{
  public static int find(int v)
  {
    if (c[v]==v) return v;
    else
    {
      c[v]=find(c[v]);
      return(c[v]);
    }
  }
  
  public static void main(String[] args) throws IOException
  {
    int n=Integer.parseInt(in.readLine());
    int[][] e=new int[n+1][2];
    for (int i=1;i<=n-1;i++)
    {
      StringTokenizer st=new StringTokenizer(in.readLine());
      e[i][0]=Integer.parseInt(st.nextToken());
      e[i][1]=Integer.parseInt(st.nextToken());
    }
    int t=0;
    int[][] anso=new int[n+1][2];
    int[][] ansn=new int[n+1][2];
    int[] size=new int[n+1];
    for (int i=1;i<=n;i++) {c[i]=i;size[i]=1;}
    for (int i=1;i<=n-1;i++)
    {
      int a=e[i][0];
      int b=e[i][1];
      int x=find(a);
      int y=find(b);
      if (x==y)
      {
        t++;
        anso[t][0]=a;anso[t][1]=b;
      }
      else
      {
        if (size[x]>size[y]) 
        {c[y]=x; size[x]+=size[y];}
        else {c[x]=y;size[y]+=size[x];}
      }
    }
    int q=0;
    t=0;
    for (int i=1;i<=n;i++)
    {
      if (c[i]==i)
      {
        if (q==0) q=i;
        else
        {
          t++;
          ansn[t][0]=q;
          ansn[t][1]=i;
          q=i;
        }
      }  
    }
    out.println(t);
    for (int i=1;i<=t;i++)
    {
      out.println(anso[i][0]+" "+anso[i][1]+" "+ansn[i][0]+" "+ansn[i][1]);
    }
    out.flush();
  }
  
  public static     int[] c=new int[1001];
  public static BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
  public static PrintWriter out=new PrintWriter(new BufferedWriter(
                                  new OutputStreamWriter(System.out))); 
}