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.*;
import static java.lang.Integer.*;
import static java.lang.Math.*;

public class Problem25D2 {
    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 Problem25D2().go();
        out.close();
    }

    int n;
    int[] parent;
    int[] sz;
    boolean[] root;
    int rootCount;

    void go() throws IOException {
        n = parseInt(in.readLine().trim());
        parent = new int[n];
        sz = new int[n];
        root = new boolean[n];
        rootCount = n;
        for (int i = 0; i < n; i++) {
            // set root as node as itself
            parent[i] = i;
            sz[i] = 1;
            root[i] = true;
        }
        ArrayList<String> extraRoads = new ArrayList<String>();
        for (int inpCount = 0; inpCount < n - 1; inpCount++) {
            String[] inp = in.readLine().trim().split("\\s+");
            int p = parseInt(inp[0]) - 1;
            int q = parseInt(inp[1]) - 1;
            int rootP = findRoot(p);
            int rootQ = findRoot(q);
            if (rootP == rootQ) {
                extraRoads.add((p + 1) + " " + (q + 1));
                continue;
            }
            // apply weighted union
            if (sz[rootP] < sz[rootQ]) {
                doUnion(rootP, rootQ);
            } else {
                doUnion(rootQ, rootP);
            }
        }
        out.println(rootCount - 1);
        if (rootCount > 1) {
            int prevRoot = -1;
            int extraRoadUsed = 0;
            for (int i = 0; i < n; i++) {
                if (root[i]) {
                    if (prevRoot == -1) {
                        prevRoot = i;
                    } else {
                        StringBuilder ans = new StringBuilder("");
                        ans.append(extraRoads.get(extraRoadUsed++));
                        ans.append(" ");
                        ans.append((prevRoot + 1) + " " + (i + 1));
                        out.println(ans);
                        prevRoot = i;
                    }
                }
            }
        }
    }

    int findRoot(int v) {
        // find root of v, and apply path compression
        for (; v != parent[v]; v = parent[v]) {
            parent[v] = parent[parent[v]];
        }
        return v;
    }

    void doUnion(int fromRoot, int toRoot) {
        parent[fromRoot] = toRoot;
        sz[toRoot] += sz[fromRoot];
        // add new root
        if (!root[toRoot]) {
            rootCount++;
            root[toRoot] = true;
        }
        // delete fromRoot
        if (root[fromRoot]) {
            rootCount--;
            root[fromRoot] = false;
        }
    }
}
