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.ArrayList;
import java.util.StringTokenizer;

/**
 * Created by mostafa on 6/22/17.
 */
public class Berland {
    static class Pair {
        int u, v;
        Pair(int a, int b) {
            u = a; v = b;
        }
    }
    static ArrayList<Integer> adjList[];
    static int n;
    static boolean[] vis;
    static void dfs(int u, ArrayList<Pair> ans) {
        vis[u] = true;
        for(int v : adjList[u])
            if(!vis[v]) {
                vis[v] = true;
                dfs(v, ans);
            }
            else {
                ans.add(new Pair(u, v));
            }
    }

    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        ArrayList<Pair> rem = new ArrayList<>();
        ArrayList<Integer> heads = new ArrayList<>();
        vis = new boolean[n];
        adjList = new ArrayList[n];
        for(int i = 0; i < n; i++)
            adjList[i] = new ArrayList<>();

        for(int i = 0; i < n - 1; i++) {
            int u = sc.nextInt() - 1, v = sc.nextInt() - 1;
            adjList[u].add(v);
            adjList[v].add(u);
        }

        for(int i = 0; i < n; i++)
            if(!vis[i]) {
                heads.add(i);
                dfs(i, rem);
            }
        PrintWriter out = new PrintWriter(System.out);

        out.println(heads.size() - 1);
        int ansIdx = 0;
        for(int i = 0; i < heads.size() - 1; i++) {
            int rem1 = rem.get(ansIdx).u + 1;
            int rem2 = rem.get(ansIdx).v + 1;
            ansIdx++;
            out.printf( "%d %d %d %d\n", rem1, rem2, heads.get(i) + 1, heads.get(i + 1) + 1);
        }

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

    static class Scanner {
        BufferedReader br;
        StringTokenizer st;

        Scanner(FileReader f) {
            br = new BufferedReader(f);
        }

        public boolean ready() throws IOException {
            return br.ready();
        }

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

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

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

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

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

}
