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.*;

public class working
{
    static PrintWriter out=new PrintWriter((System.out));
    static ArrayList<Integer>[] adj;
    static ArrayList<Integer> ans=new ArrayList<>();
    static int[] init,goal;
    public static void main(String args[])throws IOException
    {
        Reader sc=new Reader();
        
        int n=sc.nextInt();
        // adj=new ArrayList[n+1];
        // for(int i=0;i<=n;i++) adj[i]=new ArrayList<>();
        int[] par=new int[n+1];
        int[] rank=new int[n+1];
        for(int i=1;i<=n;i++) make_set(i,par,rank);
        ArrayList<int[]> del=new ArrayList<>();
        ArrayList<int[]> merge=new ArrayList<>();
        for(int tt=0;tt<n-1;tt++){
            int x=sc.nextInt(),y=sc.nextInt();
            // adj[x].add(y);
            // adj[y].add(x);
            if(find_set(x,par)==find_set(y, par)){
                del.add(new int[]{x,y});
            }
            else union_set(x, y, par, rank);
        }
        // comp
        for(int i=2;i<=n;i++){
            if(find_set(i, par)!=find_set(1, par)){
                merge.add(new int[]{1,i});
            }
        }
        out.println(del.size());
        for(int i=0;i<del.size();i++){
            out.print(del.get(i)[0]+" "+del.get(i)[1]+" ");
            out.println(merge.get(i)[0]+" "+merge.get(i)[1]);
        }       

        out.close();
    }
    // dsu
    
    private static void make_set(int v,int[] par,int[] rank){
        par[v]=v;
        rank[v]=0;
    }
    private static int find_set(int x,int[] par){
        if(par[x]==x) return x;
        return par[x]=find_set(par[x],par);
    }
    private static void union_set(int x,int y,int[] par,int[] rank){
        x=find_set(x,par);
        y=find_set(y,par);
        if(x==y) return;
        if(rank[x]<rank[y]){
            int tmp=x;
            x=y;
            y=tmp;
        }
        par[y]=x;
        if(rank[x]==rank[y]){
            rank[x]++;
        }
    }
    
    private static int dfs(int i,int par,int even_count,int odd_count,int level){
        int res=0;
        int curr;
        if(level%2==0){
            curr=(init[i]+even_count)%2;
            if(curr!=goal[i]){
                even_count++;
                res++;
                ans.add(i);
            }
        }else{
            curr=(init[i]+odd_count)%2;
            if(curr!=goal[i]){
                odd_count++;
                res++;
                ans.add(i);
            }
        }
        
        for(int j:adj[i]){
            if(j==par) continue;
            res+=dfs(j,i,even_count,odd_count,level+1);
        }
        return res;
    }
    
    // private static void dfs(ArrayList<Integer>[] adj,int i,int par,boolean[] vis){
    //     for(int j:adj[i]){
    //         if(j!=par){
    //             vis[j]=true;
    //             dfs(adj,j,i,vis);
    //         }
    //     }
    // }

    // private static boolean possible(char[][] grid,int i,int j){
    //     return (i>=0 && j>=0 && i<grid.length && j<grid[0].length);
    // }
    /*
    int n=sc.nextInt();
        	char[] s=sc.next().toCharArray();
        	int res=0;
        	for(int i=1;i<n;i++){
        		if( (s[i]=='1') && s[i]==s[i-1]) res++;
        	}
        	out.println(res);
    
    private static int modInverse(int a, int m) 
    { 
        a = a % m; 
        for (int x = 1; x < m; x++) 
           if ((a * x) % m == 1) 
              return x; 
        return 1;  
   }
    
    private static int gcd(int a,int b){
    	if(b==0) return a;
    	return gcd(b,a%b);
    }
    
    private static int dfs(ArrayList<Integer>[] adj,int[] arr,int i,int par){
    	if(adj[i].size()==0) return arr[i];
    	int curr=arr[i],s=adj[i].size();
    	int max=0;
        for(int j:adj[i]){
        	if(j==par) continue;
        	int temp=dfs(adj,arr,j,i);
        	max=Math.max(max,temp);
        	curr+=temp;
        }
        int rem=curr%s;
        if(rem>0) rem=1;
        int ans=rem+curr/s;
        if(max>ans) return max;
        return ans;
    }
     */
    public static long pow(long a,long b){
        if(b==0){
            return 1;
        }
        if(b%2==0){
            return pow(a*a,b/2);
        }else{
            return a*pow(a*a,b/2);
        }
    }
    static class Reader 
    { 
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st=new StringTokenizer("");
        public String next()
        {
            while(!st.hasMoreTokens())
            {
                try
                {
                    st=new StringTokenizer(br.readLine());
                }
                catch(Exception e)
                {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
        public int nextInt()
        {
            return Integer.parseInt(next());
        }
        public long nextLong()
        {
            return Long.parseLong(next());
        }
        public double nextDouble()
        {
            return Double.parseDouble(next());
        }
        public String nextLine()
        {
            try
            {
                return br.readLine();
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }
            return null;
        }
        public boolean hasNext()
        {
            String next=null;
            try
            {
                next=br.readLine();
            }
            catch(Exception e)
            {
            }
            if(next==null)
            {
                return false;
            }
            st=new StringTokenizer(next);
            return true;
        }
    } 
}
 