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 
// package CodeForces;

import java.io.*;
import java.util.*;

public class Problem_25D {
	public static void main(String[] args) throws IOException {
		Scanner sc = new Scanner();
		PrintWriter pw = new PrintWriter(System.out);
		int n=sc.nextInt();
		int[]a=new int[n+1];
		Arrays.fill(a,-1);
		DSU dsu=new DSU(1+n--);
		while(n-->0)
		{
			int x=sc.nextInt(),y=sc.nextInt();
			if(dsu.issameset(x, y))
				a[x]=y;
			else
				dsu.union(x, y);
		}
		int c=0;
		for(int i=1;i<a.length;i++)
		{
			if(a[i]!=-1)
			{
				for(int j=1;j<a.length;j++)
				{
					if(!dsu.issameset(i, j))
					{
						pw.println(i+" "+a[i]+" "+i+" "+j);
						c++;
						dsu.union(i, j);
						break;
					}
				}
			}
		}
		System.out.println(c);
		pw.close();
	}
	
	static class DSU
	{
		int[]p;
		int[]size;
		public DSU(int n)
		{
			p=new int[n];size=new int[n];
			for(int i=0;i<n;i++)
			{	
				p[i]=i;
				size[i]=1;
			}
		}
		public int findset(int x)
		{
			return x==p[x]?x:(p[x]=findset(p[x]));
		}
		public boolean issameset(int x,int y)
		{
			return findset(x)==findset(y);
		}
		public void union(int x,int y)
		{
			x=findset(x);y=findset(y);
			if(y==x)
				return;
			if(size[y]>=size[x])
			{
				p[x]=y;
				size[y]+=size[x];
			}
			else
			{
				p[y]=x;
				size[x]+=size[y];
			}
		}
		public String toString()
		{
			String s="";
			for(int i=0;i<p.length;i++)
			{
				s+=i+": ";
				for(int j=0;j<p.length;j++)
				{
					if(findset(j)==i)
					{
						s+=j+" ";
					}
				}
				s+="\n";
			}
			return s;
		}
	
	}
	static class Scanner {
		BufferedReader br;
		StringTokenizer st;

		Scanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() throws IOException {
			while (st == null || !st.hasMoreTokens()) {
				st = new StringTokenizer(br.readLine());
			}
			return st.nextToken();
		}

		int nextInt() throws IOException {
			return Integer.parseInt(next());
		}

		long nextLong() throws IOException {
			return Long.parseLong(next());
		}

		double nextDouble() throws IOException {
			return Double.parseDouble(next());
		}

		String nextLine() throws IOException {
			return br.readLine();
		}

		boolean hasnext() throws IOException {
			return br.ready();
		}

	}
}
