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 CF;

import java.awt.Point;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class C {

	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		int n = sc.nextInt();
		UnionFind uf = new UnionFind(n+1);
		Point [] unused = new Point[n-1];
		int idx = 0;
		for (int i = 0; i < n-1; ++i) {
			int u = sc.nextInt(), v = sc.nextInt();
			if(!uf.unionSet(u, v))
				unused[idx++] = new Point(u, v);
		}
		int res = 0;
		StringBuilder sb = new StringBuilder();
		for (int i = 2, k = 0; i <= n; ++i) {
			if(uf.unionSet(1, i)){
				++res;
				sb.append(unused[k].x + " " + unused[k++].y + " 1 " + i).append("\n");
			}
		}
		out.println(res);
		if(res > 0)
			out.println(sb);
		out.flush();
		out.close();
	}
	static class UnionFind {                                              
		int[] p, rank, setSize;
		int numSets;

		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 boolean unionSet(int i, int j) 
		{ 
			if (isSameSet(i, j)) 
				return false;
			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]++; 
			} 
			return true;
		}

		public int numDisjointSets() { return numSets; }

		public int sizeOfSet(int i) { return setSize[findSet(i)]; }
	}
	static class Scanner {
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream s) {
			br = new BufferedReader(new InputStreamReader(s));
		}

		public Scanner(FileReader fileReader) {
			br = new BufferedReader(fileReader);
		}

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

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

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

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

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