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

public class Problem25D {
    static BufferedReader in;
    static PrintWriter out;

    public static void main(String[] args) throws Exception {
        in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);
        new Problem25D().go();
        out.close();
    }

    int[][] adjList;
    int[] degree;
    boolean[] seen;
    HashSet<Point> extraRoads = new HashSet<Point>();
    ArrayList<Integer> components = new ArrayList<Integer>();

    String sp = " ";

    void go() throws IOException {
        int n = parseInt(in.readLine().trim());
        adjList = new int[n + 1][n + 1];
        degree = new int[n + 1];
        for (int i = 0; i < n - 1; i++) {
            String[] inp = in.readLine().trim().split("\\s+");
            int u = parseInt(inp[0]);
            int v = parseInt(inp[1]);
            adjList[u][degree[u]++] = v;
            adjList[v][degree[v]++] = u;
        }
        seen = new boolean[n + 1];
        for (int i = 1; i <= n; i++) {
            if (!seen[i]) {
                seen[i] = true;
                dfs(i);
                components.add(i);
            }
        }
        int countNewRoads = components.size() - 1;
        out.println(countNewRoads);
        if (countNewRoads > 0) {
            Iterator<Point> iter = extraRoads.iterator();
            for (int i = 0; i < countNewRoads; i++) {
                StringBuilder ans = new StringBuilder("");
                Point extraRoad = iter.next();
                ans.append(round(extraRoad.getX()));
                ans.append(sp);
                ans.append(round(extraRoad.getY()));
                ans.append(sp);
                ans.append(components.get(i));
                ans.append(sp);
                ans.append(components.get(i + 1));
                out.println(ans);
            }
            
        }
    }

    void dfs(int u) {
        for (int i = 0; i < degree[u]; i++) {
            int v = adjList[u][i];
            if (seen[v]) {
                extraRoads.add(new Point(min(u, v), max(u, v)));
            } else {
                seen[v] = true;
                dfs(v);
            }
        }
    }
}
