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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.StreamTokenizer;
import java.util.HashSet;
import java.util.Set;

public class D {
    
    private StreamTokenizer in;
    private PrintWriter out;
    private static final int MAX = 1005;
    private int[][] badroads = new int[4][MAX];
    private int[] heads = new int[MAX];
    private Node[] nodes = new Node[MAX];
    private Set<Integer> set = new HashSet<Integer>();
    
    public static void main (String[] args) throws IOException {
        new D().soilve();
    }
    
    int nextInt () throws IOException {
        in.nextToken();
        return (int)in.nval;
    }
    
    void soilve () throws IOException {
        in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
        out = new PrintWriter(new OutputStreamWriter(System.out));
        
        int n = nextInt();
        for (int i = 1; i <= n; i++) {
            nodes[i] = make(i);
        }
        
        int cnt = 0;
        for (int i = 1; i <= n - 1; i++) {
            int u = nextInt();
            int v = nextInt();
            if (find(nodes[u]).key != find(nodes[v]).key) {
                union(nodes[u], nodes[v]);
            } else {
                badroads[0][++cnt] = u; badroads[1][cnt] = v;
            }
        }
        
        int headIndex = 0;
        for (int i = 1; i <= n; i++) {
            if (!set.contains(find(nodes[i]).key)) {
                set.add(find(nodes[i]).key);
                heads[++headIndex] = find(nodes[i]).key;
            }
        }
        
        out.println(cnt);
        for (int i = 1; i < headIndex; i++) {
            badroads[2][i] = heads[i];
            badroads[3][i] = heads[i + 1];
        }
        
        for (int i = 1; i <= cnt; i++) {
            out.println(badroads[0][i] + " " + badroads[1][i] + " " + badroads[2][i] + " " + badroads[3][i]);
        }
        
        out.flush();
    }
    
    public class Node {
        int key, rank;
        Node p;
    }
    
    public Node make(int key) {
        Node x = new Node();
        x.key = key;
        x.rank = 0;
        x.p = x;
        return x;
    }
    
    public Node find(Node x) {
        if (x != x.p) {
            x.p = find(x.p);
        }
        return x.p;
    }
    
    public void union(Node x, Node y) {
        link(find(x), find(y));
    }
    
    public void link(Node x, Node y) {
        if (x.rank > y.rank) {
            y.p = x;
        } else {
            x.p = y;
            if (x.rank == y.rank) {
                y.rank++;
            }
        }
    }

}
