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.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Random;
import java.util.StringTokenizer;

public class Main {

    public static void main(String[] args) throws Exception {
        //System.setIn(new FileInputStream("1"));
        in = new BufferedReader(new InputStreamReader(System.in, "ISO-8859-9"));
        out = new PrintWriter(System.out);

        solve();

        in.close();
        out.close();
    }

    private static void solve() throws Exception {
        Random rand = new Random();
        int N = nextInt();
        int[] p = new int[N];
        for (int i = 0; i < N; i++) {
            p[i] = i;
        }
        int[][] extended = new int[2][N];
        int cE = 0;
        for (int i = 0; i < N - 1; i++) {
            int a = nextInt() - 1, b = nextInt() - 1;
            int x = findSet(a, p), y = findSet(b, p);
            if (x != y) {
                if (rand.nextBoolean())
                    p[x] = y;
                else
                    p[y] = x;
            } else {
                extended[0][cE] = a;
                extended[1][cE] = b;
                cE++;
            }
        }
        boolean[] components = new boolean[N];
        for (int i = 0; i < N; i++) {
            components[findSet(i, p)] = true;
        }
        out.println(cE);
        int j = 0;
        while (!components[j]) ++j;
        for (int i = 0; i < cE; i++) {
            out.print(extended[0][i] + 1);
            out.print(' ');
            out.print(extended[1][i] + 1);
            out.print(' '); 
            int bj = j++;
            while (!components[j]) ++j;
            out.print(bj + 1);
            out.print(' ');
            out.println(j + 1);
        }
    }

    private static int findSet(int a, int[] p) {
        if (a == p[a])
            return a;
        else 
            return p[a] = findSet(p[a], p);
    }

    static StringTokenizer st;
    static BufferedReader in;
    static PrintWriter out;

    static int nextInt() throws IOException {
        return Integer.parseInt(nextString());
    }

    static double nextDouble() throws IOException {
        return Double.parseDouble(nextString());
    }

    static String nextString() throws IOException {
        while (st == null || !st.hasMoreTokens()) {
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }
}
