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

public class Main {
	static Scanner f = new Scanner(System.in);
	static PrintWriter out = new PrintWriter(System.out);

	public static void main(String[] args) {
		solve();
		out.close();
	}

	static void solve() {
		int n = f.nextInt();
		ArrayList<Integer>[] arr = new ArrayList[n+1];
		Dsu dsu = new Dsu(n+1);
		for(int i=0;i<=n;i++) {
			arr[i] = new ArrayList<>();
		}

		for(int i=1;i<n;i++) {
			int u = f.nextInt(), v = f.nextInt();
			arr[u].add(v);
			arr[v].add(u);
			dsu.join(u, v);
		}


		//kinek mennyi felesleges éle van?
		TreeMap<Integer, Integer> map = new TreeMap<>();
		int[] cnt = new int[n+1];
		for(int i=1;i<=n;i++) {
			int par = dsu.getPar(i);
			cnt[par]++;
			if(map.containsKey(par)) {
				map.put(par, map.get(par) + arr[i].size());
			} else {
				map.put(par, arr[i].size());
			}
		}

		Queue<TreeSet<Pair>> many = new ArrayDeque<>();
		Queue<Integer> single = new ArrayDeque<>();
		boolean[] bol = new boolean[n+1];
		int ans = 0;
		for(Map.Entry<Integer, Integer> i : map.entrySet()) {
			int edgePlus = (i.getValue() - (cnt[dsu.getPar(i.getKey())] - 1) * 2) / 2;

			if(edgePlus == 0) {
				single.add(i.getKey());
			} else {
				Queue<int[]> qu = new ArrayDeque<>();
				qu.add(new int[] {i.getKey(), -1});
				bol[i.getKey()] = true;
				TreeSet<Pair> set = new TreeSet<>();
				while(!qu.isEmpty()) {
					int[] now = qu.poll();
					for(int j : arr[now[0]]) {
						if(now[1] != j) {
							if(bol[j]){
								set.add(new Pair(Math.min(j, now[0]), Math.max(j, now[0])));
							} else {
								bol[j] = true;
								qu.add(new int[] {j, now[0]});
							}
						}
					}
				}

				many.add(set);
				ans += set.size();
			}

		}

		out.println(ans);
		while(!many.isEmpty()) {
			TreeSet<Pair> now = many.poll();
			int last = 0;
			for(Pair p : now) {
				int next = single.poll();
				last = next;
				out.println(p.x+" "+p.y+" "+p.x+" "+next);
			}

			single.add(last);

		}



	}

	static class Pair implements Comparable<Pair> {
		int x, y;
		Pair(int x, int y) {
			this.x = x;
			this.y = y;
		}

		public int compareTo(Pair o) {
			if(o.x == x && o.y == y) {
				return 0;
			}
			int loc = Integer.compare(x, o.x);
			return loc == 0 ? Integer.compare(y, o.y) : loc;
		}

	}


	static class Dsu {
		int[] par;
		int[] size;

		Dsu(int n) {
			par = new int[n];
			size = new int[n];
			for(int i = 0;i<n;i++) {
				par[i] = i;
				size[i] = 1;
			}
		}

		int getPar(int x) {			//get parent
			if(x == par[x]) {
				return x;
			}
			return par[x] = getPar(par[x]);
		}

		void join(int a, int b) {	//join sets
			a = getPar(a);
			b = getPar(b);

			if(a != b) {
				if(size[a] < size[b]) {
					int c = a;
					a = b;
					b = c;
				}
				par[b] = a;
				size[a] += b;
			}
		}
	}
		


}