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.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author juacar01
 */
public class Roads {

    /**
     * @param args the command line arguments
     */
    // public static Nodo[] cut ;
    //public static Nodo[] nuevo ;
    public static ArrayList<Nodo> cut = new ArrayList<Nodo>();
    public static ArrayList<Nodo> nuevo = new ArrayList<Nodo>();

    public static int[] pa = new int[1010];

    public static void main(String[] args) {
        try {
            // TODO code application logic here
            int numciudades = 0;
            InputStreamReader isr = new InputStreamReader(System.in);
            BufferedReader bf = new BufferedReader(isr);
            numciudades = Integer.parseInt(bf.readLine());
            int ans = 0;

            cut = new ArrayList<Nodo>();
            nuevo = new ArrayList<Nodo>();
            for (int i = 0; i < numciudades; i++) {
                pa[i] = i;
            }

            int lciudad = 0, lb = 0;
            for (int i = 0; i < numciudades - 1; i++) {
                String[] strc = bf.readLine().split(" ");
                int a = Integer.parseInt(strc[0]), b = Integer.parseInt(strc[1]);
                if (Find(a) == Find(b)) {
                    if (ans > 0) {
                        nuevo.add(new Nodo(lciudad, b));
                        union(lciudad, b);
                    }
                    cut.add(new Nodo(a, b));
                    lciudad = a;
                    ans++;
                } else {
                    union(a, b);
                    lb = b;
                }
            }
            if (Find(lciudad) != Find(lb)) {
                nuevo.add(new Nodo(lciudad, lb));
                union(lciudad, lb);
            }

            System.out.println("" + ans);
            for (int i = 0; i < cut.size() ; i++) {
                System.out.print("" + cut.get(i).getU() + " " + cut.get(i).getV() + " " );
                if(nuevo.size()-1 >= i)
                    System.out.print(""+ nuevo.get(i).getU() + " " + nuevo.get(i).getV());
                System.out.println("");
            }
            
            /*
            int num = 0;
            for (int i = 1; i < numciudades; i++) {
                for (int j = 1; j < numciudades; j++) {
                    if (Find(i) != Find(j)) {
                        System.out.println("" + cut[num].getU() + " " + cut[num].getV() + " " + i + " " + j);
                        union(i, j);
                        num++;
                        if (num == ans) {
                            break;
                        }
                    }
                }
            }*/

        } catch (IOException ex) {
            Logger.getLogger(Roads.class.getName()).log(Level.SEVERE, null, ex);
        }

    }

    private static int Find(int x) {
        if (x != pa[x]) {
            pa[x] = Find(pa[x]);
        }
        return pa[x];
    }

    private static void union(int a, int b) {
        int x = Find(a);
        int y = Find(b);
        //nuevo.add(new Nodo(x,y));
        
        pa[x] = y;
    }

    private static class Nodo {

        private int u;
        private int v;

        public Nodo(int u, int v) {
            this.u = u;
            this.v = v;
        }

        public int getU() {
            return u;
        }

        public void setU(int u) {
            this.u = u;
        }

        public int getV() {
            return v;
        }

        public void setV(int v) {
            this.v = v;
        }

    }

}
