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.awt.event.InputEvent;
import java.io.*;
import java.util.*;

import static java.lang.Math.*;

public class Main {

    static int p[];
    static ArrayList<Integer> ans;

    static void union(int i, int j) {
        int pi = parent(i);
        int pj = parent(j);

        if (pi == pj) {
            ans.add(i);
            ans.add(j);
        } else {
            p[pi] = pj;
        }
    }

    static int parent(int i) {
        if (p[i] != i) {
            return p[i] = parent(p[i]);
        }
        return i;
    }

    static void init(int n) {
        p = new int[n];
        for (int i = 0; i < p.length; i++) {
            p[i] = i;
        }
        ans = new ArrayList<Integer>();
    }

    public static void main(String[] args) throws FileNotFoundException, AWTException, InterruptedException {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        init(n);
        for (int i = 0; i < n - 1; i++) {
            int a = in.nextInt() - 1;
            int b = in.nextInt() - 1;
            union(a, b);
        }
        System.out.println(ans.size() / 2);
        for (int k = 0; k < ans.size(); k += 2) {
            int i = ans.get(k);
            int j = ans.get(k + 1);
            int u = parent(i);
            int v = -1;
            for (int l = 0; l < n; l++) {
                if (parent(l) != u) {
                    v = l;
                    break;
                }
            }
            union(u, v);
            System.out.println(i + 1 + " " + (j + 1) + " " + (u + 1) + " " + (v + 1));
        }
    }


    static class Pair<F, S> implements Comparable<Pair<F, S>>, Comparator<Pair<F, S>> {
        F f;
        S s;
        private final Comparator<? super F> fComparator;
        private final Comparator<? super S> sComparator;

        public Pair(F f, S s, Comparator<? super F> fComparator, Comparator<? super S> sComparator) {
            this.f = f;
            this.s = s;
            this.fComparator = fComparator;
            this.sComparator = sComparator;
        }

        public Pair(F f, S s) {
            this(f, s, null, null);
        }


        @Override
        public String toString() {
            return "[" + f + ", " + s + ']';
        }

        @Override
        public int compareTo(Pair<F, S> o) {
            Comparable<? super F> fcomp = (Comparable<? super F>) f;
            Comparable<? super S> scomp = (Comparable<? super S>) s;
            if (fComparator == null) {
                return fcomp.compareTo(o.f) == 0 ? scomp.compareTo(o.s) : fcomp.compareTo(o.f);
            } else {
                return fComparator.compare(f, o.f) == 0 ? sComparator.compare(s, o.s) : fComparator.compare(f, o.f);
            }
        }

        @Override
        public int compare(Pair<F, S> o1, Pair<F, S> o2) {
            return o1.compareTo(o2);
        }
    }

}