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.*;

public class Roads2 {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    ArrayList<ArrayList<Integer>> forest = new ArrayList<ArrayList<Integer>>();
    ArrayList<Point> extra = new ArrayList<Point>();
    int[] nodeSet = new int[n];
    for(int i = 0; i < n; i++) {
      ArrayList<Integer> s = new ArrayList<Integer>();
      s.add(i);
      forest.add(s);
      nodeSet[i] = i;
    }
    for(int i = 1; i < n; i++) {
      int a = scan.nextInt() - 1;
      int b = scan.nextInt() - 1;
      if(nodeSet[a] != nodeSet[b]) {
        forest.get(nodeSet[a]).addAll(forest.get(nodeSet[b]));
        b = nodeSet[b];
        for(int x : forest.get(b)) {
          nodeSet[x] = nodeSet[a];
        }
        forest.set(b, null);
      } else {
        extra.add(new Point(a, b));
      }
    }
    System.out.println(extra.size());
    for(int i = 0; i < extra.size(); i++) {
      Point e = extra.get(i);
      System.out.print((e.x+1) + " " + (e.y+1) + " ");
      int x = 0+0; while(forest.get(x) == null) x++;
      int y = x+1; while(forest.get(y) == null) y++;
      System.out.print((forest.get(x).get(0)+1) + " ");
      System.out.println((forest.get(y).get(0)+1));
      forest.get(x).addAll(forest.get(y));
      forest.set(y, null);
    }
  }
}
