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.*;
import java.util.*;

public class RoadsnotonlyinBerland {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int n = s.nextInt(), m = 0;
        ArrayList<Integer> a = new ArrayList<>();
        ArrayList<Integer> b = new ArrayList<>();
        a.add(0);
        for (int i = 1; i < n + 1; i++) {
            a.add(i);
        }
        for (int i = 0; i < n - 1; i++) {
            int l = s.nextInt();
            int r = s.nextInt();
            if (findPar(l, a) == findPar(r, a)) {
                b.add(l);
                b.add(r);
                continue;
            }
            union(l, r, a);
        }
        int[] pairs = new int[n + 1];
        for (int i = 1; i < n + 1; i++) {
            pairs[findPar(i, a)]++;
        }
        Set<Integer> r = new HashSet<>();
        for (int i = 1; i < n + 1; i++) {
            if (pairs[i] != 0) {
                r.add(i);
            }
        }
        System.out.println(r.size() - 1);
        Integer[] rr = r.toArray(new Integer[r.size()]);
        for (int i = 0; i < b.size(); i += 2) {
            for (int ii = 0; ii < rr.length; ii++) {
                if (b.get(i) != rr[ii]) {
                    System.out.print(b.get(i) + " " + b.get(i + 1) + " ");
                    System.out.println(b.get(i) + " " + rr[ii]);
                    break;
                }
            }
        }

    }

    public static int findPar(int x, ArrayList<Integer> Par) {
        if (x == Par.get(x)) {
            return x;
        }
        Par.set(x, findPar(Par.get(x), Par));
        return Par.get(x);
    }

    public Boolean find(int x, int y, ArrayList<Integer> Par) {
        return findPar(x, Par) == findPar(y, Par);
    }

    public static void union(int x, int y, ArrayList<Integer> Par) {
        int Gparx = findPar(x, Par);
        int Gpary = findPar(y, Par);
        Par.set(Gparx, Gpary);
    }
}
