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.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.StreamTokenizer;
import java.util.Arrays;


public class Main implements Runnable {
	public static void main(String[] args) {
		new Thread(new Main()).start();
	}

	PrintWriter out;
	StreamTokenizer in;
	
	public void run() {
		try {
			in = new StreamTokenizer(new InputStreamReader(System.in));
			out = new PrintWriter(System.out);
			solve();
		} catch (Exception e) {
			e.printStackTrace();
			System.exit(1);
		} finally {
			out.close();
		}
	}
	
	int next() throws IOException {
		in.nextToken();
		return (int)in.nval;
	}
	
	void solve() throws IOException {
		n = next();
		par = new int[n];
		size = new int[n];
		for (int i = 0; i < n; ++i) {
			par[i] = i;
			size[i] = 1;
		}
		
		int[] fst, scd;
		fst = new int[n];
		scd = new int[n];
		int len = 0;
		
		for (int i = 0, a, b, f, s; i + 1 < n; ++i) {
			a = get(f = (next() - 1));
			b = get(s = (next() - 1));
			if (a == b) {
				fst[len] = f;
				scd[len] = s;
				++len;
				continue;
			}
			unite(a, b);
		}
		
		out.println(len);
		
		boolean[] h = new boolean[n];
		Arrays.fill(h, false);
		for (int i = 0; i < n; ++i)
			h[get(i)] = true;
		
		int cur = 0;
		while (!h[cur]) ++cur;
		
		for (int i = 0, prev; i < len; ++i) {
			prev = cur;
			for (cur = cur + 1; !h[cur]; ++cur);
			out.println((fst[i] + 1) + " " + (scd[i] + 1) + " " + (prev + 1) + " " + (cur + 1));		
		}
	}
	
	int n;
	int[] par, size;
	
	void unite(int a, int b) {
		if (size[a] < size[b]) {
			int t = a;
			a = b;
			b = t;
		}
		par[b] = a;
		size[a] += size[b];
	}
	
	int get(int v) {
		return v == par[v] ? v : (par[v] = get(par[v]));
	}
}
