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.math.BigInteger;
import java.util.*;
import java.io.*;
import java.util.Map.Entry;

public class Codeforces {

    static class DSU {

        int[] p, rank;
        int numSets;

        public DSU(int n) {
            p = new int[n];
            rank = new int[n];
            numSets = n;
            for (int i = 0; i < n; i++) {
                p[i] = i;
            }
        }

        int find(int x) {
            return x == p[x] ? x : (p[x] = find(p[x]));
        }

        boolean isSameSet(int i, int j) {
            return find(i) == find(j);
        }

        void union(int i, int j) {
            i = find(i);
            j = find(j);
            if (i == j) {
                return;
            } else {
                numSets--;
                if (rank[i] > rank[j]) {
                    p[j] = i;
                } else {
                    p[j] = i;
                    if (rank[i] == rank[j]) {
                        rank[j]++;
                    }
                }
            }
        }

        int getNumSets() {
            return numSets;
        }
    }

    static class Edge {
        int u, v;

        public Edge(int u, int v) {
            this.u = u;
            this.v = v;
        }

        @Override
        public String toString() {
            return u + " " + v;
        }
    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw = new PrintWriter(System.out);
        int n = Integer.parseInt(br.readLine());
        DSU dsu = new DSU(n);
        ArrayList<Edge> deleted = new ArrayList<>();
        for (int i = 0; i < n - 1; i++) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int u = Integer.parseInt(st.nextToken()) - 1;
            int v = Integer.parseInt(st.nextToken()) - 1;
            int numSets = dsu.getNumSets();
            dsu.union(u, v);
            if (numSets == dsu.getNumSets()) {
                deleted.add(new Edge(u + 1, v + 1));
            }
        }
        for(int i = 0; i < n; i++){
            dsu.find(i);
        }
        ArrayList<Edge> built = new ArrayList<>();
        TreeSet<Integer> rep = new TreeSet<>();
        for (int i = 0; i < n; i++) {
            rep.add(dsu.p[i]);
        }
        int f = rep.pollFirst();
        while (!rep.isEmpty()) {
            built.add(new Edge(f + 1, rep.pollFirst() + 1));
        }
        pw.println(deleted.size());
        for (int i = 0; i < deleted.size(); i++) {
            pw.println(deleted.get(i) + " " + built.get(i));
        }
        pw.flush();
        pw.close();
    }
}