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.ArrayDeque;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Scanner;

/**
 * Created with IntelliJ IDEA.
 * Author   : Dylan
 * Date     : 2013-08-10
 * Time     : 12:05
 * Project  : Roads not only in Berland
 */

public class Main {
    static LinkedList<Integer>[] graph;
    static int[] root;
    static ArrayDeque<Pair> extra;
    static boolean[] visited;
    static int n;

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        n = in.nextInt();
        graph = new LinkedList[n + 1];
        root = new int[n + 1];
        extra = new ArrayDeque<Pair>(n);
        for(int i = 1; i <= n; i++) {
            graph[i] = new LinkedList<Integer>();
            root[i] = i;
        }
        for(int i = 1; i < n; i++) {
            int x = in.nextInt();
            int y = in.nextInt();
            graph[x].add(y);
            graph[y].add(x);
            int rootX = findRoot(x);
            int rootY = findRoot(y);
            if(rootX != rootY) {
                root[rootY] = rootX;
            } else {
                extra.add(new Pair(x, y));
            }
        }
        System.out.println(extra.size());
        int t = findRoot(1);
        for(int i = 1; i <= n; i++) {
            int rooti = findRoot(i);
            if(t == rooti) continue;
            root[rooti] = t;
            System.out.println(extra.peek().x + " " + extra.poll().y + " 1" + " " + i);
        }

    }

    static int findRoot(int x) {
        return x == root[x] ? x : (root[x] = findRoot(root[x]));
    }
}

class Pair {
    int x, y;

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