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.ArrayList;
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));
        out = new PrintWriter(System.out);

        solve();

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

    private static void solve() throws Exception {
        int n = nextInt();
        ArrayList<Integer>[] gr = new ArrayList[n];
        for (int i = 0; i < n; i++) {
            gr[i] = new ArrayList<Integer>();
        }
        for (int i = 0; i < n - 1; i++) {
            int a = nextInt() - 1, b = nextInt() - 1;
            gr[a].add(b);
            gr[b].add(a);
        }
        ArrayList<int[]> cycleEdges = new ArrayList<int[]>();
        ArrayList<Integer> w = new ArrayList<Integer>();
        int[] use = new int[n];
        for (int i = 0; i < n; i++) {
            if (use[i] == 0) {
                dfs(i, -1, gr, use, cycleEdges);
                w.add(i);
            }
        }
        out.println(cycleEdges.size());
        int i = 0;
        for (int[] e : cycleEdges) {
            out.print(e[0] + 1);
            out.print(' ');
            out.print(e[1] + 1);
            out.print(' ');
            out.print(e[1] + 1);
            out.print(' ');
            out.println(w.get(i + 1) + 1);
            i++;
        }
    }

    private static void dfs(int i, int prev, ArrayList<Integer>[] gr, int[] use,
            ArrayList<int[]> cycleEdges) {
        use[i] = 1;
        for (int j : gr[i]) {
            if (j == prev)
                continue;
            if (use[j] == 0)
                dfs(j, i, gr, use, cycleEdges);
            else if (use[j] == 1)
                cycleEdges.add(new int[] {i, j});
        }
        use[i] = 2;
    }

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

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

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

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