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.*;

public class d25 {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		int n = scan.nextInt();
		DSU dsu = new DSU(n);
		ArrayDeque<pair> useless = new ArrayDeque<>();
		for (int i = 0; i < n - 1; ++i) {
			int u = scan.nextInt() - 1;
			int v = scan.nextInt() - 1;
			boolean didNothing = dsu.unite(u, v);
			if (didNothing) {
				useless.add(new pair(u, v));
			}
		}
		ArrayList<Integer> roots = new ArrayList<>();
		for (int i = 0; i < n; ++i) {
			if (dsu.isRoot(i)) {
				roots.add(i);
			}
		}
		System.out.println(roots.size() - 1);
		for (int i = 0; i < roots.size() - 1; ++i) {
			pair toRemove = useless.poll();
			int u = roots.get(i);
			int v = roots.get(i + 1);
			System.out.printf("%d %d %d %d\n",
				toRemove.u + 1, toRemove.v + 1, u + 1, v + 1);
		}
	}

	static class pair {
		int u;
		int v;

		pair(int a, int b) {
			u = a;
			v = b;
		}
	}

	static class DSU {
		int n;
		int[] parent, size;

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

		public boolean isRoot(int curr) {
			return curr == parent[curr];
		}

		public int findRoot(int curr) {
			if (curr == parent[curr])
				return curr;
			return parent[curr] = findRoot(parent[curr]);
		}

		/** returns true if already in the same union */
		public boolean unite(int a, int b) {
			int rootA = findRoot(a);
			int rootB = findRoot(b);
			if (rootA == rootB)
				return true;
			if (size[rootA] > size[rootB]) {
				parent[rootB] = rootA;
				size[rootA] += size[rootB];
			} else {
				parent[rootA] = rootB;
				size[rootB] += size[rootA];
			}
			return false;
		}
	}
}