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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class D {

    static private int [] parent = new int [1005];
    static private int [] rank = new int [1005];
    public static void main(String[] args) throws IOException
    {
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        List<Integer> L1 = new ArrayList<>(), L2 = new ArrayList<>();

        int n = in.nextInt();
        for(int i=1;i<=n;i++){
            makeSet(i);
        }

        for(int i=0;i<n-1;i++) {
            int x = in.nextInt(), y = in.nextInt();
            boolean t = unionSets(x, y);
            if(!t) {
                L1.add(x);
                L2.add(y);
            }
        }
        out.println(L1.size());
        for(int i = 2, j = 0;i<=n;i++){
            if(findSet(i) != findSet(1)){
                out.println(L1.get(j) + " " + L2.get(j) + " " + 1 + " " +  i );
                unionSets(1,i);
                j++;
            }
        }

        out.flush();
        out.close();

    }

    private static void makeSet(int x)
    {
          parent[x] = x;
          rank[x] = 0;
    }

    private static int findSet(int x)
    {
        if( x == parent[x] ) return x;
        return  parent[x] = findSet(parent[x]);
    }

    private static boolean unionSets(int a, int b)
    {
        a = findSet(a);
        b = findSet(b);
        if( a == b) return false;
        if(rank[a] < rank[b]) {
            int temp = a;
            a = b;
            b = temp;
        }
        parent[b] = a;
        if(rank[b] == rank[a]) rank[a]++;
        return  true;

    }

    static class Scanner
    {
        StringTokenizer st;
        BufferedReader br;

        public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}

        public String next() throws IOException
        {
            while (st == null || !st.hasMoreTokens())
                st = new StringTokenizer(br.readLine());
            return st.nextToken();
        }

        public int nextInt() throws IOException {return Integer.parseInt(next());}

        public long nextLong() throws IOException {return Long.parseLong(next());}

        public String nextLine() throws IOException {return br.readLine();}

        public double nextDouble() throws IOException { return Double.parseDouble(next()); }
    }
}
