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.text.*;
import java.math.*;
import static java.lang.Integer.*;
import static java.lang.Double.*;
import java.lang.Math.*;

public class roads_not_only_in_berland {

	public static FastIO file = new FastIO();

	public static int ntc, ctc;
	public static final long MOD = 1000000007L; // 998244353L;
	public static final int N = 200005, dx[] = { 0, 1, 0, -1 }, dy[] = { 1, 0, -1, 0 };

	public void run() throws Exception {
		ntc = 1; // nextInt();
		for (ctc = 1; ctc <= ntc; ++ctc)
			solve();
		file.out.flush();
		file.out.close();
	}

	public static class DSU {
		int[] pset;
		int[] size;
		int numsets;

		public DSU(int size) {
			this.numsets = size;
			this.pset = new int[size];
			this.size = new int[size];
			for (int i = 0; i < size; i++)
				this.pset[i] = i;
			for (int i = 0; i < size; i++)
				this.size[i] = 1;
		}

		public int find_set(int i) {
			return pset[i] == i ? i : (pset[i] = find_set(pset[i]));
		}

		public void unify(int i, int j) {
			int a = find_set(i), b = find_set(j);
			if (a == b)
				return;
			numsets--;
			if (size[a] < size[b]) {
				pset[a] = b;
				size[b] += size[a];
			} else {
				pset[b] = a;
				size[a] += size[b];
			}
		}

		public boolean same_set(int i, int j) {
			return find_set(i) == find_set(j);
		}

		int size_set(int i) {
			return size[find_set(i)];
		}
	}

	public static void solve() {
		int n = nextInt();
		DSU d = new DSU(n);
		ArrayList<Pair<Integer, Integer>> unne = new ArrayList<>(), add = new ArrayList<>();
		for (int i = 0; i < n - 1; i++) {
			int u = nextInt() - 1, v = nextInt() - 1;
			if (d.same_set(u, v)) {
				unne.add(new Pair<Integer, Integer>(u + 1, v + 1));
			}
			d.unify(u, v);
		}

		for (int j = 1; j < n; j++) {
			if (!d.same_set(0, j)) {
				add.add(new Pair<Integer, Integer>(1, j + 1));
				d.unify(0, j);
			}
		}

	println(add.size());
		//System.out.println(add);
		for (int i = 0; i < add.size(); i++) {
			println(unne.get(i).fi + " " + unne.get(i).se + " " + add.get(i).fi + " " + add.get(i).se);
		}
	}

	public static long mod(long n, long mod) {
		return (n % mod + mod) % mod;
	}

	public static long pow(long n, long p, long mod) {
		long ret = 1L;
		while (p > 0) {
			if (p % 2 != 0L)
				ret = mod(ret * n, mod);
			n = mod(n * n, mod);
			p >>= 1L;
		}
		return ret;
	}

	public static long pow(long n, long p) {
		long ret = 1L;
		while (p > 0) {
			if (p % 2 != 0L)
				ret *= n;
			n *= n;
			p >>= 1L;
		}
		return ret;
	}

	public static long gcd(long x, long y) {
		while (x != 0) {
			long tmp = x;
			x = y % x;
			y = tmp;
		}
		return y;
	}

	public static long lcm(long x, long y) {
		return x / gcd(x, y) * y;
	}

	public static boolean isPrime(int n) {
		if (n <= 1)
			return false;
		if (n <= 3)
			return true;
		if (n % 2 == 0 || n % 3 == 0)
			return false;
		for (int i = 5; i * i <= n; i = i + 6)
			if (n % i == 0 || n % (i + 2) == 0)
				return false;
		return true;
	}

	public static class Pair<A, B> implements Comparable {
		public A fi;
		public B se;

		public Pair(A fi, B se) {
			this.fi = fi;
			this.se = se;
		}

		@Override
		public boolean equals(Object o) {
			if (this == o)
				return true;
			if (o == null || getClass() != o.getClass())
				return false;
			Pair<?, ?> p = (Pair<?, ?>) o;
			if (!fi.equals(p.fi))
				return false;
			return se.equals(p.se);
		}

		@Override
		public int hashCode() {
			return 31 * fi.hashCode() + se.hashCode();
		}

		@Override
		public String toString() {
			return fi.toString() + " " + se.toString();
		}

		public static <A, B> Pair<A, B> of(A a, B b) {
			return new Pair<A, B>(a, b);
		}

		public int compareTo(Object o) {
			Pair<?, ?> p = (Pair<?, ?>) o;
			if (fi.equals(p.fi))
				return ((Comparable) se).compareTo(p.se);
			return ((Comparable) fi).compareTo(p.fi);
		}
	}

	public static String next() {
		return file.next();
	}

	public static int nextInt() {
		return file.nextInt();
	}

	public static long nextLong() {
		return file.nextLong();
	}

	public static double nextDouble() {
		return file.nextDouble();
	}

	public static String nextLine() {
		return file.nextLine();
	}

	public static void print(Object o) {
		file.out.print(o);
	}

	public static void println(Object o) {
		file.out.println(o);
	}

	public static void printf(String s, Object... o) {
		file.out.printf(s, o);
	}

	public static class FastIO {
		BufferedReader br;
		StringTokenizer st;
		PrintWriter out;

		public FastIO() {
			br = new BufferedReader(new InputStreamReader(System.in));
			out = new PrintWriter(System.out);
		}

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

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

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

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

		String nextLine() {
			String str = "";
			try {
				str = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

		void print(Object o) {
			out.print(o);
		}

		void println(Object o) {
			out.println(o);
		}

		void printf(String s, Object... o) {
			out.printf(s, o);
		}
	}

	public static void main(String[] args) throws Exception {
		new roads_not_only_in_berland().run();
	}
}
