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 
//package algoEx;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class RoadsnotonlyinBerland {
//25D 5.25
	  static Scanner scan = new Scanner(System.in);
	    static int x, y, a, b;
	    static int n = scan.nextInt();
	    static int[] t = new int[n+1];

	    public static int find(int temp)
	    {
	        if (t[temp] == temp)
	            return t[temp];
	        else
	            return find(t[temp]);
	    }

	    public static void main(String[] args)
	    {
	        List<Pair<Integer, Integer>> close = new ArrayList<Pair<Integer, Integer>>();
	        List open = new ArrayList<Integer>();
	        for (int i = 1; i <= n; i++)
	            t[i] = i;
	        for (int i = 1; i < n; i++)
	        {
	            x = scan.nextInt();
	            y = scan.nextInt();
	            a = find(x);
	            b = find(y);
	            if (a == b)
	                close.add(Pair.of(x, y));
	            else
	                t[Math.max(a, b)] = Math.min(a, b);
	        }
	        for (int i = 1; i <= n; i++)
	            if (t[i] == i)
	                open.add(i);
	        System.out.println(close.size());
	        for (int i = 0; i < close.size(); i++)
	        {
	            System.out.println();
	            System.out.print(close.get(i).first + " " + close.get(i).second + " ");
	            System.out.print(open.get(0) + " " + open.get(i + 1));
	        }
	    }
	}

	class Pair<L, R>
	{
	    L first;
	    R second;

	    public Pair(L a, R b)
	    {
	        this.first = a;
	        this.second = b;
	    }

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

}
