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.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;

public class Main {

    private PrintWriter out;

    private Edge[] edges;
    private int n;
    
    class Edge {
        public int start, end;
        public Edge(int start, int end) {
            this.start = start;
            this.end = end;
        }
    }
    
    class DSU {
        
        int[] p;
        int[] size;
        
        public DSU(int n) {
            p = new int[n+1];
            size = new int[n+1];
            for (int i=0; i<=n; i++) {
                p[i] = i;
                size[i] = 1;
            }
        }
        
        public int find(int i) {
            if (p[i] == i) {
                return i;
            }
            return p[i] = find(p[i]);
        }
        
        public void union(int a, int b) {
            a = find(a); b = find(b);
            if (size[a] > size[b]) {
                size[a] += size[b];
                p[b] = a;
            } else {
                p[a] = b;
                size[b] += size[a];
            }
        }
        
    }
    
    private void solve() throws Exception {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(new OutputStreamWriter(System.out));

        n = Integer.valueOf(in.readLine());
        edges = new Edge[n];
        for (int i=0; i<n-1; i++) {
            StringTokenizer st = new StringTokenizer(in.readLine());
            edges[i] = new Edge(Integer.valueOf(st.nextToken()), Integer.valueOf(st.nextToken()));
        }
        
        DSU dsu = new DSU(n);
        List<Edge> redundantEdges = new ArrayList<Edge>(); 
        for (int i=0; i<n-1; i++) {
            if (dsu.find(edges[i].start) != dsu.find(edges[i].end)) {
                dsu.union(edges[i].start, edges[i].end);
            } else {
                redundantEdges.add(edges[i]);
            }
        }
        
        System.out.println(redundantEdges.size());
        int j = 0;
        for (int i=2; i<=n; i++) {
            if (dsu.find(1) != dsu.find(i)) {
                dsu.union(1, i);
                System.out.println(edges[j].start + " " + edges[j++].end + " 1 " + i);
            }
        }
        
        
        out.flush();
        out.close();
    }

    public static void main(String[] args) throws Exception {
        new Main().solve();
    }
}
