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.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class D {
    static int dsu[] , n,a,b;

    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.flush();
    }
    static class TaskB{
        void solve(int test , InputReader in , PrintWriter out) {
            n = in.nextInt();
            dsu = new int[1001];
            for (int i = 1; i <= n; i++)
                dsu[i] = i;
            
            ArrayList<pair> rem = new ArrayList();
            	for (int i = 0; i < n-1; ++i){
		a = in.nextInt();
		b = in.nextInt();
		if(find(a) == find(b)){
			rem.add(new pair(a,b));
		}else{
			dsu[find(a)] = dsu[find(b)];
		}
	}
                
            ArrayList<Integer> g = new ArrayList();
            for (int i = 1; i <= n; i++) {
                if(find(i) == i)
                    g.add(i);
            }
            
            out.println(g.size()-1);
            for (int i = 0; i < g.size()-1; i++) {
                out.printf("%d %d %d %d\n", rem.get(i).first , rem.get(i).second , g.get(i),g.get(i+1));
            }
//            out.printf("%d %d %d %d\n", rem.get(0).first , rem.get(0).second , g.get(0),5);
            
            }
            int find(int u){
                if(dsu[u] == u)
                    return u;
                 return dsu[u] = find(dsu[u]);
            }
    }
        public static class pair{
        int first , second;
        pair(int first,int second){
            this.first = first;
            this.second = second;
        }
    }
       static class InputReader{
       public BufferedReader reader;
       public StringTokenizer tk;
       public InputReader(InputStream stream){
           reader = new BufferedReader(new InputStreamReader(stream));
           tk = null;
       }
       String next(){
           while (tk == null || !tk.hasMoreTokens()) {
               try{
                   tk = new StringTokenizer(reader.readLine());
               }
               catch(IOException e){
                   throw new RuntimeException();
               }
           }
           return tk.nextToken();
       }
       int nextInt(){
           return Integer.parseInt(next());
       }
       long nextLong(){
           return Long.parseLong(next());
       }        
   } 
}
