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.io.PrintWriter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Random;
import java.util.Scanner;
import java.util.TreeSet;

import javafx.util.Pair;

public class Main {

    static Scanner in = new Scanner(System.in);
    static PrintWriter out = new PrintWriter(System.out);
    static int root[];
    static ArrayList<Pair<Integer, Integer>> unUsedRoute = new ArrayList<>();
    static Random rand = new Random();

    static void makeSet(int u) {
        root[u] = u;
    }

    static int getRoot(int u) {
        if (u == root[u]) {
            return u;
        }
        root[u] = getRoot(root[u]);
        return root[u];
    }

    static void mergeUnion(int u, int v) {
        int rootU = getRoot(u);
        int rootV = getRoot(v);
        if (rootU != rootV)
            if (rand.nextInt(2) % 2 == 0)
                root[rootV] = rootU;
            else
                root[rootU] = rootV;
        else
            unUsedRoute.add(new Pair<>(u + 1, v + 1));
    }
    public static void main(String[] args) {
        int n = in.nextInt();
        root = new int[n];
        for (int i = 0; i < n; i++) {
            makeSet(i);
        }

        for (int i = 0; i < n - 1; i++) {
            Integer u = in.nextInt() - 1;
            Integer v = in.nextInt() - 1;
            mergeUnion(u, v);
        }

        TreeSet<Integer> disjointUnion = new TreeSet<>();
        for (int i = 0; i < n; i++) {
            disjointUnion.add((getRoot(i) + 1));
        }
            
        Object disjointUnionList[] = disjointUnion.toArray();
        out.println(unUsedRoute.size());
        int index = 0;
        for (Pair<Integer, Integer> entry : unUsedRoute) {
            out.println(entry.getKey() + " " + entry.getValue() + " " + disjointUnionList[index] + " " + disjointUnionList[index + 1]);
            index++;
        }

        in.close();
        out.close();
    }
}