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.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.security.KeyStore.Entry;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Scanner;
import java.util.Stack;

public class Frinda {
	static class UnionFind {
		int[] p, rank;

		UnionFind(int N) {
			p = new int[N];
			rank = new int[N];
			for (int i = 0; i < N; i++)
				p[i] = i;
		}

		int findSet(int x) {
			return p[x] == x ? x : (p[x] = findSet(p[x]));
		}

		boolean isun(int x, int y) {
			return findSet(x) == findSet(y);
		}

		boolean union(int x, int y) {
			x = findSet(x);
			y = findSet(y);
			if (x == y)
				return false;

			if (rank[x] > rank[y])
				p[y] = x;
			else {
				p[x] = y;
				if (rank[x] == rank[y])
					++rank[y];
			}
			return true;
		}

	}

	public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		Scanner sc = new Scanner(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		int n = sc.nextInt();

		UnionFind uf = new UnionFind(n);
		Stack<point> del =new Stack<>();
		for (int i = 1; i < n; i++) {
			int a=sc.nextInt()-1;
			int b=sc.nextInt()-1;
			if(uf.isun(a, b))
				del.add(new point (a,b));
			else
				uf.union(a, b);
		}
		int c = del.size();
		pw.println(c);
		for(int i=1;i<n;i++) {
			if(!uf.isun(0, i)) {
				uf.union(0, i);
				point p=del.pop();
				pw.println((p.x+1)+" "+(p.y+1)+" "+1+" "+(i+1));
			}
		}

		pw.flush();

	}
	static class point {
		public int x;
		public int y;

		public point(int f, int g) {
			x = f;
			y = g;
		}
	}

}
