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.*;
import java.util.ArrayList;

public class CF25D {
    static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    static PrintWriter pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    static int[] rank, parent;
    
    public static void main(String[] args) throws IOException {
        int numCities = Integer.parseInt(br.readLine());
        rank = new int[numCities];
        parent = new int[numCities];
        
        for(int i = 0; i < parent.length; i++)
            parent[i] = i;
        
        String[] split;
        ArrayList<String> delete = new ArrayList<String>();
        ArrayList<String> create = new ArrayList<String>();
        
        for(int i = 1; i < numCities; i++) {
            split = br.readLine().split(" ");
            int u = Integer.parseInt(split[0]) - 1;
            int v = Integer.parseInt(split[1]) - 1;
            
            int x = findSet(u);
            int y = findSet(v);
            
            if(x == y) delete.add((u + 1) + " " + (v + 1));
            else unionSet(x, y);
        }
        
        for(int i = 1; i < numCities; i++) {
            int x = findSet(i);
            int y = findSet(i - 1);
            
            if(x != y) {
                create.add((i) + " " + (i + 1));
                unionSet(x, y);
            }
        }
        
        pw.println(delete.size());
        for(int i = 0; i < delete.size(); i++) {
            pw.println(delete.get(i) + " " + create.get(i));
        }
        
        pw.close();
    }
    public static int findSet(int i) {
        if(parent[i] == i) return i;
        parent[i] = findSet(parent[i]);
        return parent[i];
    }
    public static void unionSet(int i, int j) {
        int x = findSet(i);
        int y = findSet(j);
        
        if(rank[x] < rank[y]) parent[x] = y;
        else {
            parent[y] = x;
            if(rank[x] == rank[y])
                rank[x]++;
        }
    }
}