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 

// Submitted By Subhash Yadav
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.*;

public class Main {

	// DSU
	static int parent[];
	static int size[];
	static int n;

	public static void intialize() {
		parent = new int[n];
		size = new int[n];
		for (int i = 0; i <  n; i++) {
			parent[i] = i;
			size[i] = 1;
		}

	}

	// find parent of the set
	public static int find_set(int a) {
		return parent[a] = (a == parent[a] ? a : find_set(parent[a]));

	}

	// combine the set
	public static void union(int a, int b) {
		a = find_set(a);
		b = find_set(b);
		if (a != b) {
		if(size[a]<size[b]) {
			int t=a;
			a=b;
			b=t;
		}
		
		parent[b]=a;
		size[a]+=size[b];
		
		
		}
	}

	

	public static void main(String[] args) throws IOException {

		FastScanner sc = new FastScanner();
		n = sc.nextInt();
		
		ArrayList<int []> close=new ArrayList<>();
		ArrayList<int []> add=new ArrayList<>();
		intialize();
		
		for(int i=0;i<n-1;i++) {
			int x=sc.nextInt()-1,y=sc.nextInt()-1;
			if(find_set(x)==find_set(y)) {
				int a1[]= {x,y};
				close.add(a1);
			}
			else {
				union(x, y);
			}
		}
		
		for(int i=1;i<n;i++) {
			if(find_set(i)!=0) {
				union(i, 0);
				int a1[]= {i,0};
				add.add(a1);
			}
		}
		System.out.println(close.size());
		for(int i=0;i<close.size();i++) {
			System.out.println((close.get(i)[0]+1)+" "+(close.get(i)[1]+1)+" "+(add.get(i)[0]+1)+" "+(add.get(i)[1]+1));
		}

	}

	static class FastScanner {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st = new StringTokenizer("");

		String next() {
			while (!st.hasMoreTokens())
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}

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

		int[] readArray(int n) {
			int[] a = new int[n];
			for (int i = 0; i < n; i++)
				a[i] = nextInt();
			return a;
		}

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

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

}