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.util.*;
import java.io.*;
public class RoadsnotonlyinBerland {
    
    
    
    /************************ SOLUTION STARTS HERE ************************/
    

    static class DSU {
        private int parent[];
        private int size[];
        private int cnt;

        DSU(int length) {
            this.cnt = length;
            parent = new int[length + 10];
            size = new int[length + 10];
            Arrays.fill(size, 1);
            for (int i = 0; i < parent.length; i++)
                parent[i] = i;
        }

        int root(int p) {
            while(p != parent[p]) p = parent[p];
            return p;
        }

        int sizeOf(int p) {
            return size[root(p)];
        }

        boolean connected(int u, int v) {
            return root(u) == root(v);
        }

        int components() {
            return cnt;
        }

        void union(int u, int v) {
            if (!connected(u, v)) {
                cnt--;
                int rootU = root(u);
                int rootV = root(v);
                if (size[rootU] < size[rootV]) {
                    parent[rootU] = rootV;
                    size[rootV] += size[rootU];
                } else {
                    parent[rootV] = rootU;
                    size[rootU] += size[rootV];
                }
            }
        }
    }
    
    
    private static void solve() {
        
        
        int V = nextInt();
        int E = V - 1;
        
        DSU unionFind = new DSU(V);
        ArrayList<int[]> toRemove = new ArrayList<>();
        
        HashSet<Integer> cycleComps = new HashSet<>();
        TreeSet<Integer> treeComps = new TreeSet<>();
        
        while(E-->0) {
            int u = nextInt();
            int v = nextInt();
            if(unionFind.connected(u, v)) 
                toRemove.add(new int[]{u, v});
            unionFind.union(u, v);
        }
        
        for(int p[] : toRemove)
            cycleComps.add(unionFind.root(p[0]));
        
        for(int i = 1; i <= V; i++) {
            int root = unionFind.root(i);
            if(!cycleComps.contains(root))
                treeComps.add(root);
        }
        
//        println(treeComps);
//        println(cycleComps);
        
        println(toRemove.size());
        for(int p[] : toRemove) {
            int other = treeComps.isEmpty() ? toRemove.get(0)[0] : treeComps.pollFirst();
            println(p[0] + " " + p[1] + " " + p[0] + " " + other);
        }
        
    }
    
    
    
    /************************ SOLUTION ENDS HERE ************************/
    
    
    
    
    
    /************************ TEMPLATE STARTS HERE **********************/
    
    public static void main(String[] args) throws IOException {
        reader = new BufferedReader(new InputStreamReader(System.in));
        writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)), false);
        st     = null;
        solve();
        reader.close();
        writer.close();
    }
    
    static BufferedReader reader;
    static PrintWriter    writer;
    static StringTokenizer st;
    
    static String next()
    {while(st == null || !st.hasMoreTokens()){try{String line = reader.readLine();if(line == null){return null;}            
    st = new StringTokenizer(line);}catch (Exception e){throw new RuntimeException();}}return st.nextToken();}
    static String nextLine()  {String s=null;try{s=reader.readLine();}catch(IOException e){e.printStackTrace();}return s;}             
    static int    nextInt()   {return Integer.parseInt(next());}
    static long   nextLong()  {return Long.parseLong(next());}     
    static double nextDouble(){return Double.parseDouble(next());}
    static char   nextChar()  {return next().charAt(0);}
    static int[]  nextIntArray(int n)         {int[] a= new int[n];   int i=0;while(i<n){a[i++]=nextInt();}  return a;}
    static long[] nextLongArray(int n)        {long[]a= new long[n];  int i=0;while(i<n){a[i++]=nextLong();} return a;}    
    static int[]  nextIntArrayOneBased(int n) {int[] a= new int[n+1]; int i=1;while(i<=n){a[i++]=nextInt();} return a;}            
    static long[] nextLongArrayOneBased(int n){long[]a= new long[n+1];int i=1;while(i<=n){a[i++]=nextLong();}return a;}            
    static void   print(Object o)  { writer.print(o);  }
    static void   println(Object o){ writer.println(o);}
    
    /************************ TEMPLATE ENDS HERE ************************/
    
}