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.HashMap;
import java.util.HashSet;
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();
        HashSet<Integer>[] gr = new HashSet[n];
        for (int i = 0; i < n; i++) {
            gr[i] = new HashSet<Integer>();
        }
        for (int i = 0; i < n - 1; i++) {
            int a = nextInt() - 1;
            int b = nextInt() - 1;
            gr[a].add(b);
            gr[b].add(a);
        }
        HashSet<HashSet<Integer>> components = new HashSet<HashSet<Integer>>();
        boolean[] use = new boolean[n];
        for (int i = 0; i < n; i++) {
            if (!use[i]) {
                HashSet<Integer> cmp = new HashSet<Integer>();
                dfs(i, gr, cmp, use);
                components.add(cmp);
            }
        }
        ArrayList<int[]> ans = new ArrayList<int[]>();
        while (components.size() > 1) {
            HashSet<Integer> cFrom = getFrom(components, gr);
            HashSet<Integer> cTo = getTo(components, gr);
            
            if (cFrom == null || cTo == null)
                break;
            
            int fromVertex = maxDeg(cFrom, gr);
            int deVertex = delV(cFrom, gr, fromVertex);
            int toVertex = maxDeg(cTo, gr);
            
            gr[fromVertex].remove(deVertex);
            gr[deVertex].remove(fromVertex);
            gr[fromVertex].add(toVertex);
            gr[toVertex].add(fromVertex);
            
            ans.add(new int[] {fromVertex, deVertex, fromVertex, toVertex});
            cFrom.addAll(cTo);
            components.remove(cTo);
        }
        out.println(ans.size());
        for (int[] v : ans) {
            for (int i = 0; i < v.length - 1; i++) {
                out.print(v[i] + 1);
                out.print(' ');
            }
            out.println(v[v.length - 1]);
        }
    }

    private static int delV(HashSet<Integer> cFrom, HashSet<Integer>[] gr,
            int fromVertex) {
        for (int v : gr[fromVertex]) {
            int sum = 0;
            for (int j : gr[v]) {
                if (cFrom.contains(j))
                    ++sum;
            }
            if (sum > 1)
                return v;
        }
        return -1;
    }


    private static int maxDeg(HashSet<Integer> cFrom, HashSet<Integer>[] gr) {
        int ind = -1, best = -1;
        for (int v : cFrom) {
            if (best < gr[v].size()) {
                ind = v;
                best = gr[v].size();
            }
        }
        return ind;
    }

    private static HashSet<Integer> getTo(HashSet<HashSet<Integer>> components,
            HashSet<Integer>[] gr) {
        for (HashSet<Integer> it : components) {
            int sum = calcSum(it, gr);
            if (sum == 2 * (it.size() - 1))
                return it;
        }
        return null;
    }

    private static HashSet<Integer> getFrom(
            HashSet<HashSet<Integer>> components, HashSet<Integer>[] gr) {
        for (HashSet<Integer> it : components) {
            int sum = calcSum(it, gr);
            if (sum != 2 * (it.size() - 1))
                return it;
        }
        return null;
    }

    private static int calcSum(HashSet<Integer> it, HashSet<Integer>[] gr) {
        int sum = 0;
        for (int v : it) {
            sum += gr[v].size();
        }
        return sum;
    }

    private static void dfs(int i, HashSet<Integer>[] gr,
            HashSet<Integer> cmp, boolean[] use) {
        use[i] = true;
        cmp.add(i);
        for (int j : gr[i]) {
            if (!use[j])
                dfs(j, gr, cmp, use);
        }
        
    }

    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());
    }
}
