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.*;
 
public class Roads_25D {
    public static void main(String[] args) throws Exception {
	new Solve().solve();
    }

    static class Solve {
	InputStream inputStream = System.in;
	OutputStream outputStream = System.out;
	InputReader in = new InputReader(inputStream);
	PrintWriter out = new PrintWriter(outputStream);

	int n;
	DSU dsu;
	List<String> close = new ArrayList<String>();
	List<String> open = new ArrayList<String>();
	Set<Integer> roots = new LinkedHashSet<Integer>();

	public void fill() {
	    n = in.nextInt();
	    dsu = new DSU(n);
	    for (int i = 1; i<=n; i++) {
		dsu.makeSet(i);
	    }

	    for (int i = 1; i<=n-1; i++) {
		int u = in.nextInt();
		int v = in.nextInt();
		if (dsu.findSet(u) == dsu.findSet(v)) {
		    close.add(String.format("%d %d", u, v));
		} else {
		    dsu.unionSets(u, v);
		}
	    }
	}

	public void solve() {
	    fill();
	    for (int i = 1; i<=n; i++) {
		roots.add(dsu.findSet(i));
	    }
	    int u = 0, v;
	    System.out.println(close.size());
	    
	    for (Integer i : roots) {
		v = i;
		if (u !=0) {
		    open.add(String.format("%d %d", u, v));
		}
		u = v;
	    }
	    for (int i = 0; i < open.size(); i++) {
		System.out.println(close.get(i) + " " + open.get(i));
	    }
	}
	
    }

    static class DSU {
	int[] parent, size; // size is only stored for the root node of each disjoint set

	public DSU(int n) {
	    this.parent = new int[n+1];
	    this.size = new int[n+1];
	}

	public void makeSet(int i) {
	    parent[i] = i;
	    size[i] = 1;
	}

	public int findSet(int i) {
	    if (parent[i] == i) {
		return i;
	    }
	    parent[i] = findSet(parent[i]);
	    return parent[i];
	}

	public void unionSets(int i, int j) {
	    int si= findSet(i), sj = findSet(j);
	    if (si == sj) {
		return;
	    }
	    if (size[si] < size[sj]) {
		int tmp = si;
		si = sj;
		sj = tmp;
	    }
	    parent[sj] = si;
	    size[si] += size[sj];
	}
    }
 
    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;
 
        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }
 
        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
 
        public int nextInt() {
            return Integer.parseInt(next());
        }
 
    }
 
 
}
