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 Chapter2;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class RoadsNotOnlyInBerland {
	static class pair{
		int one, two;
		public pair(int one, int two) {
			this.one = one;
			this.two = two;
		}
	}
	static class UnionFind {                                              
		int[] p, rank, setSize;
		int numSets;
		ArrayList<Integer> heads =new ArrayList<Integer>();
		public void findHeads() {
			for (int i = 0; i < p.length; i++) {
				if(p[i]==i)
					heads.add(i);
			}
		}
		public UnionFind(int N) 
		{
			p = new int[numSets = N];
			rank = new int[N];
			setSize = new int[N];
			for (int i = 0; i < N; i++) {  p[i] = i; setSize[i] = 1; }
		}

		public int findSet(int i) { return p[i] == i ? i : (p[i] = findSet(p[i])); }

		public boolean isSameSet(int i, int j) { return findSet(i) == findSet(j); }

		public void unionSet(int i, int j) 
		{ 
			if (isSameSet(i, j)) 
				return;
			numSets--; 
			int x = findSet(i), y = findSet(j);
			if(rank[x] > rank[y]) { p[y] = x; setSize[x] += setSize[y]; }
			else
			{	p[x] = y; setSize[y] += setSize[x];
				if(rank[x] == rank[y]) rank[y]++; 
			} 
		}

		public int numDisjointSets() { return numSets; }

		public int sizeOfSet(int i) { return setSize[findSet(i)]; }
	}
	public static void main(String[] args) throws IOException{
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(in.readLine());	
		pair [] pairs = new pair[n-1];
		UnionFind ds = new UnionFind(n);
		int numSets = n;
		boolean [] bad = new boolean[n-1];
		for (int i = 0; i < pairs.length; i++) {
			StringTokenizer st = new StringTokenizer(in.readLine());
			int one = Integer.parseInt(st.nextToken())-1;
			int two = Integer.parseInt(st.nextToken())-1;
			pairs[i]=new pair(one,two);
			ds.unionSet(one, two);
			if(ds.numSets==numSets)
				bad[i]=true;
			numSets = ds.numSets;
		}
		PrintWriter out =new PrintWriter(System.out);
		if(ds.numSets==1) {
			out.println(0);
		}else {
			ds.findHeads();
			int count = 0;
			ArrayList<Integer> heads = ds.heads;
			int con = heads.get(count++);
			out.println(numSets-1);
			for (int i = 0; i < bad.length; i++) {
				if(bad[i]) {
					out.println((pairs[i].one+1)+" "+(pairs[i].two+1)+" "+(con+1)+" "+(heads.get(count++)+1));
				}
			}
		}
	out.flush();out.close();
	}
}
