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 RnB {
	
	static int[] union;
	public static int find( int x ) {
		return union[x] == x ? x : ( union[x] = find( union[x] ) );
	}
	
	public static void main( String[] args ) throws IOException {
		BufferedReader br = new BufferedReader( new InputStreamReader( System.in ) );
		PrintWriter pw = new PrintWriter( new BufferedWriter( new OutputStreamWriter( System.out ) ) );
		
		int N = Integer.parseInt( br.readLine() );
		
		ArrayList<Integer>[] map = new ArrayList[N];
		for(int i = 0; N > i; i++) {
			map[i] = new ArrayList<Integer>();
		}
		
		for(int i = 0; N - 1 > i; i++) {
			String[] line = br.readLine().split(" ");
			int l = Integer.parseInt(line[0]) - 1;
			int r = Integer.parseInt(line[1]) - 1;
			
			map[l].add(r);
			map[r].add(l);
		}
		
		union = new int[N];
		for(int i = 0; N > i; i++) {
			union[i] = i;
		}
		
		LinkedList<Integer> starters = new LinkedList<Integer>();
		ArrayList<Integer> l = new ArrayList<Integer>();
		ArrayList<Integer> r = new ArrayList<Integer>();
		HashSet<String> set = new HashSet<String>();
		
		for(int i = 0; N > i; i++) {
			if( find(i) != i ) {
				continue;
			} //System.out.println( "STARTING WITH " + (i+1) );
			
			starters.push( i );
			
			LinkedList<S> stack = new LinkedList<S>();
			for(int j = 0; map[i].size() > j; j++) {
				stack.push( new S( map[i].get(j), i ) );
			}
			
			while( stack.size() > 0 ) {
				S s = stack.pop();
				
				int curr = find( s.index );
				int prev = find( s.prev );
				
				if( curr != prev ) {
					union[curr] = prev;
				} else {
					//System.out.println( "CYCLE WITH " + (s.index+1) + " & " + (s.prev+1) );
					String ss = Math.max( s.index, s.prev ) + " " + Math.min( s.index, s.prev );
					if( !set.contains( ss ) ) {
						l.add( s.index );
						r.add( s.prev );
						set.add( ss );
					}
					continue;
				}
				
				for(int j = 0; map[s.index].size() > j; j++) {
					int next = map[s.index].get(j);
					if( next == s.prev ) {
						continue;
					}
					
					stack.push( new S( next, s.index ) );
				}
			}
		} //System.out.println( Arrays.toString( union ) );
		
		pw.println( l.size() );
		
		/*while( starters.size() > 0 ) { System.out.print( starters.pop() + " " ); }
		System.out.println();
		System.out.println( l );
		System.out.println( r );*/
		
		for(int i = 0; l.size() > i; i++) {
			while( true ) {
				int x = starters.pop();
				if( find( x ) != find( l.get(i) ) ) {
					pw.print( l.get(i) + 1 );
					pw.print( " " );
					pw.print( r.get(i) + 1 );
					pw.print( " " );
					pw.print( l.get(i) + 1 );
					pw.print( " " );
					pw.println( x + 1 );
					break;
				} else {
					starters.offer( x );
				}
			}
		}
		
		pw.close();
	}
}

class S {
	public int index, prev;
	public S( int index, int prev ) {
		this.index = index;
		this.prev = prev;
	}
}