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.*;
import java.io.*;
import java.awt.Point;
import static java.lang.Math.*;

public class CF25D {
    static int[] uf;
    static void mix(int a, int b) {
        uf[find(a)] = find(b);
    }
    static int find(int x) {
        if(uf[x] == x) return uf[x];
        return uf[x] = find(uf[x]);
    }
    public static void main(String[] args) throws Exception {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        uf = new int[n];
        for(int i=0; i<n; i++) uf[i] = i;

        List<int[]> A = new ArrayList<int[]>();
        for(int i=0; i<n-1; i++) {
            int s = in.nextInt()-1;
            int t = in.nextInt()-1;
            if(find(s) == find(t)) {
                A.add(new int[]{s,t});
            } else {
                mix(s,t);
            }
        }
        TreeSet<Integer> X = new TreeSet<Integer>();
        for(int i=0; i<n; i++) X.add(find(i));

        System.out.println(X.size()-1);
        int ix = 0;
        while(X.size() > 1) {
            int u = X.pollFirst();
            int v = X.pollFirst();
            System.out.printf("%d %d %d %d\n", A.get(ix)[0]+1, A.get(ix)[1]+1, u+1, v+1);
            mix(u,v);
            ix++;
            X.add(u);    
        }
    }
}
