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.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;


public class RoadsNotInBerlin {

	public static void main(String[] args) throws IOException {
		Reader in = new Reader();
		PrintWriter out = new PrintWriter(System.out,true);
		int a,b,n = in.nextInt();
		ArrayList<DisjointSetNode<Integer>> roads = new ArrayList<DisjointSetNode<Integer>>(n+1);
		ArrayList<int[]> cycle = new ArrayList<int[]>(n);
		for(int i=0;i<=n;i++)roads.add(new DisjointSetNode<Integer>(new Integer(i)));
		DisjointSetNode<Integer> city1,city2;
		int t = n-1;
		int[] temp;
		for(int i=1;i<n;i++){
			a = in.nextInt(); b = in.nextInt();
			city1 = roads.get(a); city2 = roads.get(b);
			if(city1.find()!=city2.find()){
				city1.union(city2); t--;
			}
			else{
				//a cycle found
				temp = new int[2];	temp[0] = a; temp[1] = b;
				cycle.add(temp);
			}
		}
		out.println(t);
		// cycles == t
		String str = "";
		ArrayList<DisjointSetNode<Integer>> roots = new ArrayList<DisjointSetNode<Integer>>(t);
		for(DisjointSetNode<Integer> x:roads)
			if(x.parent == x) roots.add(x);
		int rootIndex = 1;
		for(int i=0;i<cycle.size();i++){
			a = cycle.get(i)[0]; b = cycle.get(i)[1];
			city1 = roads.get(a); city1 = roads.get(b);
			if(city1.find()!=roots.get(rootIndex)){
				str+=a+" "+b+" "+a+" "+roots.get(rootIndex++)+'\n';
				rootIndex %= roots.size();
			}
			else{
				city1 = roots.get(rootIndex);
				roots.set(rootIndex, roots.get((rootIndex+1)%roots.size()));
				i--;
			}
		}
		out.println(str);
	}

}
class DisjointSetNode<T>{
	T data;
	int rank;
	DisjointSetNode<T> parent,next;
	public DisjointSetNode(T d,DisjointSetNode<T> p){
		parent = p;
		rank = 0;
		data = d;
		next = this;
	}
	public DisjointSetNode(T d){
		this(d,null);
		parent = this;
	}
	public DisjointSetNode<T> find(){
		if(parent!=this){
			parent = parent.find();
		}
		return parent;
	}
	public void union(DisjointSetNode<T> x){
		DisjointSetNode<T> a,b;
		a = find();
		b = x.find();
		if(a==b)return;
		if(a.rank<b.rank){
			a.parent = b;
		}
		else if(b.rank<a.rank){
			b.parent = a;
		}
		else{
			a.rank++;
			b.parent = a;
		}
		//merge CLL
		DisjointSetNode<T> temp = a.next;
		a.next = b.next;
		b.next = temp;
	}
	public void printSet(){
		DisjointSetNode<T> curr = this;
		String str = "";
		do{
			str+=curr;
			curr = curr.next;					
		}
		while(curr!=this);
		System.out.println(str);
	}
	public String toString(){
		/*
		String str = "";
		str+="{"+rank+","+data+"} ";
		return str;
		*/
		return data.toString();
	}
}

/** Faster input **/

class Reader {
    final private int BUFFER_SIZE = 1 << 16;private DataInputStream din;private byte[] buffer;private int bufferPointer, bytesRead;
    public Reader(){din=new DataInputStream(System.in);buffer=new byte[BUFFER_SIZE];bufferPointer=bytesRead=0;}
    public Reader(String file_name) throws IOException{din=new DataInputStream(new FileInputStream(file_name));buffer=new byte[BUFFER_SIZE];bufferPointer=bytesRead=0;}
    public String readLine() throws IOException{byte[] buf=new byte[64];int cnt=0,c;while((c=read())!=-1){if(c=='\n')break;buf[cnt++]=(byte)c;}return new String(buf,0,cnt);}
    public int nextInt() throws IOException{int ret=0;byte c=read();while(c<=' ')c=read();boolean neg=(c=='-');if(neg)c=read();do{ret=ret*10+c-'0';}while((c=read())>='0'&&c<='9');if(neg)return -ret;return ret;} 
    public long nextLong() throws IOException{long ret=0;byte c=read();while(c<=' ')c=read();boolean neg=(c=='-');if(neg)c=read();do{ret=ret*10+c-'0';}while((c=read())>='0'&&c<='9');if(neg)return -ret;return ret;}
    public double nextDouble() throws IOException{double ret=0,div=1;byte c=read();while(c<=' ')c=read();boolean neg=(c=='-');if(neg)c = read();do {ret=ret*10+c-'0';}while((c=read())>='0'&&c<='9');
        if(c=='.')while((c=read())>='0'&&c<='9')ret+=(c-'0')/(div*=10);if(neg)return -ret;return ret;}
    private void fillBuffer() throws IOException{bytesRead=din.read(buffer,bufferPointer=0,BUFFER_SIZE);if(bytesRead==-1)buffer[0]=-1;}
    private byte read() throws IOException{if(bufferPointer==bytesRead)fillBuffer();return buffer[bufferPointer++];}
    public void close() throws IOException{if(din==null) return;din.close();}
}