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;
import java.util.TreeSet;

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();
    }

    static class Pair implements Comparable<Pair>{
        int deg;
        int vertex;
        
        public Pair(int deg, int vertex) {
            this.deg = deg;
            this.vertex = vertex;
        }

        @Override
        public int compareTo(Pair o) {
            if (o.deg != deg)
                return o.deg - deg;
            return vertex - o.vertex;
        }
    }
    
    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<TreeSet<Pair>> components = new ArrayList<TreeSet<Pair>>();
        boolean[] use = new boolean[n];
        for (int i = 0; i < n; i++) {
            if (!use[i]) {
                TreeSet<Pair> comp = new TreeSet<Main.Pair>();
                dfs1(i, gr, use, comp);
                components.add(comp);
            }
        }
        ArrayList<int[]> ans = new ArrayList<int[]>();
        if (components.size() == 1) {
            out.println(0);
            return;
        }
        use = new boolean[components.size()];
        while (true) {
            int index1 = -1;
            int index2 = -1;
            // where
            for (int i = 0; i < components.size(); i++) {
                if (use[i])
                    continue;
                
                int sum = 0;
                for (Pair p : components.get(i)) {
                    sum += p.deg;
                }
                if (sum == components.get(i).size() * 2 - 2) {
                    index1 = i;
                    break;
                }
            }
            // from
            for (int i = 0; i < components.size(); i++) {
                if (use[i] || i == index1)
                    continue;
                int sum = 0;
                for (Pair p : components.get(i)) {
                    sum += p.deg;
                }
                if (sum != components.get(i).size() * 2 - 2) {
                    index2 = i;
                    break;
                }
            }
            if (index1 == -1 || index2 == -1)
                break;
            
            int f2 = components.get(index2).first().vertex;
            int t2 = gr[f2].get(0);
            int t1 = components.get(index1).first().vertex;
            ans.add(new int[] {f2, t2, f2, t1});
            
            // update
            gr[f2].remove(0);
            gr[t1].add(f2);
            gr[f2].add(t1);
            for (int j = 0; j < gr[t2].size(); j++) {
                if (gr[t2].get(j) == f2) {
                    gr[t2].remove(j);
                    break;
                }
            }
            // update set
            Pair old = components.get(index1).first();
            Pair nw = new Pair(old.deg + 1, old.vertex);
            components.get(index1).remove(old);
            components.get(index1).add(nw);
            
            for (Pair p : components.get(index2)) {
                if (p.vertex == t2) {
                    old = p;
                    break;
                }
            }
            nw = new Pair(old.deg - 1, old.vertex);
            components.get(index2).remove(old);
            components.get(index2).add(nw);
            
            components.get(index2).addAll(components.get(index1));
            use[index1] = true;
        }
        out.println(ans.size());
        for (int[] v : ans) {
            for (int j = 0; j < v.length - 1; j++) {
                out.print(v[j] + 1);
                out.print(' ');
            }
            out.println(v[v.length - 1] + 1);
        }
    }

    private static void dfs1(int i, ArrayList<Integer>[] gr, boolean[] use, TreeSet<Pair> comp) {
        use[i] = true;
        Pair ans = new Pair(gr[i].size(), i);
        for (int j : gr[i]) {
            if (!use[j]) {
                dfs1(j, gr, use, comp);
            }
        }
        comp.add(ans);
    }

    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());
    }
}
