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 
        			/*
        		     * Author- Priyam Vora
        		     * BTech 2nd Year DAIICT
        		     */
     
     
        		    import java.awt.Point;
import java.io.*;
        		    import java.math.*;
        		    import java.util.*;
        		    import javax.print.attribute.SetOfIntegerSyntax;
     
     
     
        		    public class Main{
        		    	private static InputStream stream;
        		    	private static byte[] buf = new byte[1024];
        		    	private static int curChar;
        		    	private static int numChars;
        		    	private static SpaceCharFilter filter;
        		    	private static PrintWriter pw;
        		    	private static long count = 0,mod=1000000007;
        		    //	private static TreeSet<Integer> ts=new TreeSet[200000];
     
        		        public static void main(String[] args) {
        		        	InputReader(System.in);
        		    		pw = new PrintWriter(System.out); 
        		            new Thread(null ,new Runnable(){
        		               public void run(){
        		                   try{
        		                       soln2();
        		                       pw.close();
        		                   } catch(Exception e){
        		                       e.printStackTrace();
        		                   }
        		               }
        		           },"1",1<<26).start();
        		       }

        		    	public static long gcd(long x, long y) {
        		    		if (x == 0)
        		    			return y;
        		    		else
        		    			return gcd( y % x,x);
        		    	}
        		        static LinkedList<Integer> adj[];
        		        static boolean Visited[];
        		        static long color[];
        		        static long dist[][];
        		        static long no_sp_no_sub[];
        		        static long ans=0;

        		        private static void soln2(){
        		        	int n=nextInt();
        		            Dsu ds=new Dsu(n);
        		            ArrayList<Point> close=new ArrayList<>(),open=new ArrayList<>();
        		             for (int i=1;i<n;i++){
        		                int a=nextInt(),b=nextInt();
        		                if (!ds.union(a-1,b-1)) close.add(new Point(a,b));
        		            }
        		            for (int i=1;i<n;i++)
        		                if (ds.union(i,0))
        		                    open.add(new Point(1,i+1));
        		            pw.println(close.size());
        		            for (int i=0;i<close.size();i++)
        		                pw.println(close.get(i).x+" "+close.get(i).y+" "+open.get(i).x+" "+open.get(i).y);
        		        	
        		        }
        		    	private static int dfs(int x){
        		    		Visited[x]=true;
        		    		for(int i:adj[x]){
        		    		if(!Visited[i]){
        		    			dfs(i);
        		    		}
        		    		}
        		    		return x;
        		    	}
           		    	private static void buildgraph(int n){
        		    		adj=new LinkedList[n+1];
        		    		Visited=new boolean[n+1];
        		    		color=new long[n+1];
        		    	
        		    		no_sp_no_sub=new long[n+1];
        		    		ans=0;
        		    		Arrays.fill(color, 0);
        		    		for(int i=0;i<=n;i++){
        		    			adj[i]=new LinkedList<Integer>();
        		    		}
     
     
        		    	}
     
        		    	// To Get Input
        		    	// Some Buffer Methods
     
        		    	public static void InputReader(InputStream stream1) {
        		    		stream = stream1;
        		    	}
     
        		    	private static boolean isWhitespace(int c) {
        		    		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        		    	}
     
        		    	private static boolean isEndOfLine(int c) {
        		    		return c == '\n' || c == '\r' || c == -1;
        		    	}
     
        		    	private static int read() {
        		    		if (numChars == -1)
        		    			throw new InputMismatchException();
        		    		if (curChar >= numChars) {
        		    			curChar = 0;
        		    			try {
        		    				numChars = stream.read(buf);
        		    			} catch (IOException e) {
        		    				throw new InputMismatchException();
        		    			}
        		    			if (numChars <= 0)
        		    				return -1;
        		    		}
        		    		return buf[curChar++];
        		    	}
     
        		    	private static int nextInt() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		int sgn = 1;
        		    		if (c == '-') {
        		    			sgn = -1;
        		    			c = read();
        		    		}
        		    		int res = 0;
        		    		do {
        		    			if (c < '0' || c > '9')
        		    				throw new InputMismatchException();
        		    			res *= 10;
        		    			res += c - '0';
        		    			c = read();
        		    		} while (!isSpaceChar(c));
        		    		return res * sgn;
        		    	}
     
        		    	private static long nextLong() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		int sgn = 1;
        		    		if (c == '-') {
        		    			sgn = -1;
        		    			c = read();
        		    		}
        		    		long res = 0;
        		    		do {
        		    			if (c < '0' || c > '9')
        		    				throw new InputMismatchException();
        		    			res *= 10;
        		    			res += c - '0';
        		    			c = read();
        		    		} while (!isSpaceChar(c));
        		    		return res * sgn;
        		    	}
     
        		    	private static String nextToken() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		StringBuilder res = new StringBuilder();
        		    		do {
        		    			res.appendCodePoint(c);
        		    			c = read();
        		    		} while (!isSpaceChar(c));
        		    		return res.toString();
        		    	}
     
        		    	private static String nextLine() {
        		    		int c = read();
        		    		while (isSpaceChar(c))
        		    			c = read();
        		    		StringBuilder res = new StringBuilder();
        		    		do {
        		    			res.appendCodePoint(c);
        		    			c = read();
        		    		} while (!isEndOfLine(c));
        		    		return res.toString();
        		    	}
     
        		    	private static int[] nextIntArray(int n) {
        		    		int[] arr = new int[n];
        		    		for (int i = 0; i < n; i++) {
        		    			arr[i] = nextInt();
        		    		}
        		    		return arr;
        		    	}
     
        		    	private static int[][] next2dArray(int n, int m) {
        		    		int[][] arr = new int[n][m];
        		    		for (int i = 0; i < n; i++) {
        		    			for (int j = 0; j < m; j++) {
        		    				arr[i][j] = nextInt();
        		    			}
        		    		}
        		    		return arr;
        		    	}
        		    	private static char[][] nextCharArray(int n,int m){
        		    		char [][]c=new char[n][m];
        		    		for(int i=0;i<n;i++){
        		    			String s=nextLine();
        		    			for(int j=0;j<s.length();j++){
        		    				c[i][j]=s.charAt(j);
        		    			}
        		    		}
        		    		return c;
        		    	}
     
        		    	private static long[] nextLongArray(int n) {
        		    		long[] arr = new long[n];
        		    		for (int i = 0; i < n; i++) {
        		    			arr[i] = nextLong();
        		    		}
        		    		return arr;
        		    	}
     
        		    	private static void pArray(int[] arr) {
        		    		for (int i = 0; i < arr.length; i++) {
        		    			pw.print(arr[i] + " ");
        		    		}
        		    		pw.println();
        		    		return;
        		    	}
     
        		    	private static void pArray(long[] arr) {
        		    		for (int i = 0; i < arr.length; i++) {
        		    			pw.print(arr[i] + " ");
        		    		}
        		    		pw.println();
        		    		return;
        		    	}
     
        		    	private static void pArray(boolean[] arr) {
        		    		for (int i = 0; i < arr.length; i++) {
        		    			pw.print(arr[i] + " ");
        		    		}
        		    		pw.println();
        		    		return;
        		    	}
     
        		    	private static boolean isSpaceChar(int c) {
        		    		if (filter != null)
        		    			return filter.isSpaceChar(c);
        		    		return isWhitespace(c);
        		    	}
     
        		    	private interface SpaceCharFilter {
        		    		public boolean isSpaceChar(int ch);
        		    	}
     
     
    }
        		    
        		    class Node{
        		    	int to;
        		    	long dist;
        		    	Node(int to,long dist){
        		    		this.to=to;
        		    		this.dist=dist;
        		    	}
        		    	
        		    }
        			class Pair implements Comparable<Pair>{
						
						int city,pop;
						Pair(int city,int pop){
							this.city=city;
							this.pop=pop;
						}
						@Override
						public int compareTo(Pair o) {
				return pop-this.pop;
				
						}
						public int hashCode() {
		    	    			int hu = (int) (city ^ (city >>> 32));
		    	    			int hv = (int) (pop ^ (pop >>> 32));
		    	    			//int hw = (int) (mass ^ (mass >>> 32));
		    	    			return 31 * hu + hv ;
		    	    		//return 0;
		    				}
		    				public boolean equals(Object o) {
		    	    			Pair other = (Pair) o;
		    	    			return city == other.city && pop == other.pop;
		    	    		}
					}  class Dsu{
	    		    	private int rank[], parent[] ,n;
	    		    	private static int[] parent1;
	    		    	Dsu(int size){
	    		    		this.n=size+1;
	    		    		rank=new int[n];
	    		    		//parent=new int[n];
	    		    		parent=new int[n];
	    		    	makeSet();
	    		
	    		    	}
	    		    	
	    		    	void makeSet(){
	    		    		for(int i=0;i<n;i++){
	    		    			parent[i]=i;
	    		    		}
	    		    	}
	    		    	
	    		    	int find(int x){
	    		    		if(parent[x]!=x){
	    		    			
	    		    			parent[x]=find(parent[x]);
	    		    		}
	    		    		return parent[x];
	    		    	}
	    		    
	    		    	
	    		    	boolean union(int x,int y){
	    		    		int xRoot=find(x);
	    		    		int yRoot=find(y);
	    		    		
	    		    		if(xRoot==yRoot)
	    		    			return false;
	    		    		if(rank[xRoot]<rank[yRoot]){
	    		    		parent[xRoot]=yRoot;	
	    		    		}else if(rank[yRoot]<rank[xRoot]){
	    		    			parent[yRoot]=xRoot;
	    		    		}else{
	    		    			parent[yRoot]=xRoot;
	    		    			rank[xRoot]++;
	    		    		}
	    		    		return true;
	    		    	}

	    		     
	    		    }