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 Solution {
    static class UnionFind
{
	public int[] parent;
	public int[] rank;
	public int[] numberOfVertices;// number of vertices under this vertex
	public int dS;

	public UnionFind(int n)
	{
		parent=new int[n];
		rank=new int[n];
		numberOfVertices=new int[n];
		for(int i=0;i<n;i++)
			parent[i]=i;
		Arrays.fill(numberOfVertices,1);
		dS=n;
	}

	public int find(int x)
	{
		if(parent[x]==x) return x;
		return parent[x]=find(parent[x]);
	}

	public void unite(int a,int b)
	{
		int pa=this.find(a),pb=this.find(b);
		if(pa!=pb)
		{
			if(rank[pa]==rank[pb])
			{
				parent[pa]=pb;
				rank[pb]++;
				numberOfVertices[pb]+=numberOfVertices[pa];
			}
			else if(rank[pa]>rank[pb])
			{
				parent[pb]=pa;
				numberOfVertices[pa]+=numberOfVertices[pb];
			}
			else
			{
				parent[pa]=pb;
				numberOfVertices[pb]+=numberOfVertices[pa];
			}
			dS--;
		}
	}
}

    static class Kattio extends PrintWriter {
    public Kattio(InputStream i) {
        super(new BufferedOutputStream(System.out));
        r = new BufferedReader(new InputStreamReader(i));
    }
    public Kattio(InputStream i, OutputStream o) {
        super(new BufferedOutputStream(o));
        r = new BufferedReader(new InputStreamReader(i));
    }

    public boolean hasMoreTokens() {
        return peekToken() != null;
    }

    public int getInt() {
        return Integer.parseInt(nextToken());
    }

    public double getDouble() {
        return Double.parseDouble(nextToken());
    }

    public long getLong() {
        return Long.parseLong(nextToken());
    }

    public String getWord() {
        return nextToken();
    }



    private BufferedReader r;
    private String line;
    private StringTokenizer st;
    private String token;

    private String peekToken() {
        if (token == null)
            try {
                while (st == null || !st.hasMoreTokens()) {
                    line = r.readLine();
                    if (line == null) return null;
                    st = new StringTokenizer(line);
                }
                token = st.nextToken();
            } catch (IOException e) { }
        return token;
    }

    private String nextToken() {
        String ans = peekToken();
        token = null;
        return ans;
    }
}
    public static void main(String[] args) throws IOException{
        Kattio sc=new Kattio(System.in);
        int n=sc.getInt();
        UnionFind uf=new UnionFind(n);
        ArrayList<Integer> a=new ArrayList<Integer>(n);
        ArrayList<Integer> b=new ArrayList<Integer>(n);
        for(int i=0;i<n-1;i++)
        {
            int x=sc.getInt()-1;
            int y=sc.getInt()-1;
            if(uf.find(x)==uf.find(y))
            {
                a.add(x);
                b.add(y);
            }
            else
                uf.unite(x,y);
        }
        HashSet<Integer> parents=new HashSet<Integer>(n);
        for(int i=0;i<n;i++)
            parents.add(uf.find(i));
        ArrayList<Integer> parents2=new ArrayList<Integer>(parents.size());
        for(int x : parents)
            parents2.add(x);
        System.out.println(parents2.size()-1);
        for(int i=0;i<parents2.size()-1;i++)
        {
            System.out.print((a.get(i)+1)+" "+(b.get(i)+1)+" ");
            System.out.println((parents2.get(i)+1)+" "+(parents2.get(i+1)+1));
        }
    }
}