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.awt.*;

public class D
{
    static ArrayList[] e;
    static boolean[] b;
    static int cu, cv, gv;
    static boolean cyc;
    
    public static void main(String args[])
    {
        Scanner scan = new Scanner(System.in);
        
        int n = scan.nextInt();
        
        e = new ArrayList[n];
        for(int i=0;i < n;i++)
            e[i] = new ArrayList<Integer>();
        
        for(int i=0;i < n-1;i++)
        {
            int u = scan.nextInt() - 1;
            int v = scan.nextInt() - 1;
            e[u].add(v);
            e[v].add(u);
        }
        
        ArrayList<Point> rtn = new ArrayList<Point>();
        
        while(true)
        {
            b = new boolean[n];
            cu = cv = -1;
            for(int i=0;i < n;i++)
            {
                if(b[i])
                    continue;
                    
                cyc = false;
                dfs(i, -1);
                
                if(!cyc)
                    gv = i;
            }
            
            if(cu == -1)
                break;
                
            rtn.add(new Point(cu, cv));
            rtn.add(new Point(cu, gv));
            
            e[cu].remove(e[cu].indexOf(cv));
            e[cv].remove(e[cv].indexOf(cu));
            e[cu].add(gv);
            e[gv].add(cu);
        }
        
        System.out.println(rtn.size() / 2);
        for(int i=0;i < rtn.size();i++)
        {
            System.out.print((rtn.get(i).x+1) + " " + (rtn.get(i).y+1));
            
            if(i % 2 == 0)
                System.out.print(" ");
            else
                System.out.println();
        }
    }
    
    
    public static void dfs(int u, int f)
    {
        for(int i=0;i < e[u].size();i++)
        {
            int v = (Integer)e[u].get(i);
            if(f == v)
                continue;
                
            if(b[v])
            {
                cu = u;
                cv = v;
                cyc = true;
                return;
            }
            
            b[v] = true;
            dfs(v, u);
        }
    }
}
