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.util.*;

public class DisjointSetUnion {
    static class Pair {
        int x, y;

        Pair(int x, int y) {
            this.x = x;
            this.y = y;
        }
    }

    static class dsu {
        static int parent[], rank[], n;// min[],value[];

        dsu(int n) {
            parent = new int[n + 1];
            rank = new int[n + 1];
            // min=new int[n+1];
            // value=new int[n+1];
            this.n = n;
            makeSet();
        }

        static void makeSet() {
            for (int i = 1; i <= n; i++) {
                parent[i] = i;
                rank[i] = 0;
                // min[i]=i;
            }
        }

        static int find(int a) {
            if (parent[a] == a)
                return a;
            else {
                return parent[a] = find(parent[a]);// Path Compression
            }
        }

        static void union(int a, int b) {
            int setA = find(a);
            int setB = find(b);
            if (setA == setB)
                return;
            if (rank[setA] >= rank[setB]) {
                parent[setB] = setA;
                if (rank[setA] == rank[setB]) {
                    rank[setA]++;
                    rank[setB] = 0;
                }
            } else {
                parent[setA] = setB;
                rank[setA] = 0;
            }
        }
    }

    public static void main(String[] args) {
        Scanner f = new Scanner(System.in);
        int n;
        n = f.nextInt();
        ArrayList<ArrayList<Integer>> g = new ArrayList<>();
        ArrayList<Pair> roads = new ArrayList<>();
        dsu ob = new dsu(n);
        for (int i = 0; i <= n; i++) {
            g.add(new ArrayList<>());
        }
        for (int i = 0; i < n - 1; i++) {
            int u, v;
            u = f.nextInt();
            v = f.nextInt();
            g.get(u).add(v);
            g.get(v).add(u);
            if (ob.find(u) == ob.find(v)) {
                roads.add(new Pair(u, v));
            }
            ob.union(u, v);
        }
        HashMap<Integer, Integer> hm = new HashMap<>();
        for (int i = 1; i <= n; i++) {
            int repre = ob.find(i);
            if (hm.get(repre) == null) {
                hm.put(repre, 1);
            }
        }
        ArrayList<Integer> repreSet = new ArrayList<>(hm.keySet());
        System.out.println(roads.size());
        System.out.println(repreSet);
        // System.out.println(roads);
        int hash[]=new int[n+1];
        for (Pair p : roads) {
            int u = p.x;
            int v = p.y;
            int i = v;
            for (int j=0;j<repreSet.size();j++) {
                // System.out.println(ob.find(v)+" "+repreSet.get(j));
                int find=ob.find(v);
                if (find != repreSet.get(j) && hash[repreSet.get(j)]==0) {
                    System.out.print(u + " " + v + " " + i + " " + repreSet.get(j) + "\n");
                    // repreSet.set(j, -1);
                    hash[repreSet.get(j)]=1;
                    hash[find]=1;
                    break;
                }
            }
        }
    }
}