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.*;
import java.util.*;
public class DSUBerland
{
    static int parent[] = new int[1001];
    static int size [] = new int [1001];
    public static void main(String[]args)
    {
        InputReader1 sc = new InputReader1(System.in);
        int N = sc.nextInt();
        for(int i=1;i<=N;i++)
        {
            parent [i] = i;
            size[i] = 1;
        }
        ArrayList<Pair> rem = new ArrayList<Pair>();
        ArrayList<Pair> ins = new ArrayList<Pair>();
        for(int i=1;i<N;i++)
        {
            int u = sc.nextInt();
            int v = sc.nextInt();
            if(find(u) == find(v))
                rem.add(new Pair(u,v));
            else
                union(u,v);
        }
        
        for(int i=1;i<=N;i++)
        {
            for(int j=i+1;j<=N;j++)
            {
                if(find(i)!=find(j))
                {
                    ins.add(new Pair(i,j));
                    union(i,j);
                }
            }
        }
        System.out.println(ins.size());
        for(int i=0;i<ins.size();i++)
        System.out.println(rem.get(i).first+" "+rem.get(i).second+" "+ins.get(i).first+" "+ins.get(i).second);     
    }
    static int find(int x)
    {
        if(parent[x]==x)return x;
        else return parent[x] = find(parent[x]);
    }
    static void union(int i , int j)
    {
        int u = find(i);
        int v = find(j);
        if(u==v)return;
        if(size[u] < size[v])
        {
            size[v] += size[u];
            parent[u]=v;
        }
        else
        {
            size[u]+=size[v];
            parent[v]=u;
        }
    }
}
class Pair
{
    int first , second;
    public Pair(int first ,int second)
    {
        this.first = first;
        this.second = second;
    }
}
class InputReader1 
{

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;

    public InputReader1(InputStream stream) {
        this.stream = stream;
    }

    public int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public int nextInt() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c & 15;
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public long nextLong() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        long res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c & 15;
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public String next() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public String nextLine() {
        int c = read();
        //while (c != '\n' && c != '\r' && c != '\t' && c != -1)
        //c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (c != '\n' && c != '\r' && c != '\t' && c != -1);
        return res.toString();
    }

    public static boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

}  