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.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class point {
	
	public static void main(String args[]) throws IOException {
		Scanner zizo=new Scanner (System.in);
		PrintWriter wr=new PrintWriter(System.out);
		
		int n=zizo.nextInt();
		DSU dsu=new DSU(n);
		ArrayList<Integer>u=new ArrayList<>();
		ArrayList<Integer>v=new ArrayList<>();
				
		for(int i=0;i<n-1;i++) {
			int x=zizo.nextInt()-1;
			int y=zizo.nextInt()-1;
			if(dsu.sameSet(x, y)) {
				u.add(x);
				v.add(y);
			}
			dsu.union(x, y);
		}
		System.out.println(u.size());
		int idx=0;
		for(int i=0;i<n;i++)
			for(int j=i+1;j<n;j++)
				if(!dsu.sameSet(i,j)) {
					wr.println((u.get(idx)+1)+" "+(v.get(idx++)+1)+" "+(i+1)+" "+(j+1));
					dsu.union(i, j);
				}
		
		wr.close();
	}
}

class DSU{
	int[]p,size;
	DSU(int n){
		p=new int[n];
		size=new int[n];
		for(int i=0;i<n;i++) {
			p[i]=i;size[i]=1;
		}
	}
	int findSet(int x) {
		return p[x]==x?x:(p[x]=findSet(p[x]));
	}
	void union(int u,int v) {
		int x=findSet(u);
		int y=findSet(v);
		if(x==y)return;
		if(size[x]>=size[y]) {
			p[y]=x;size[x]+=size[y];
		}else {
			p[x]=y;size[y]+=size[x];
		}
	}
	boolean sameSet(int x,int y) {
		return findSet(x)==findSet(y);
	}
}

class Scanner {
	StringTokenizer st;
	BufferedReader br;

	public Scanner(InputStream s) {
		br = new BufferedReader(new InputStreamReader(s));
	}

	public String next() throws IOException {
		while (st == null || !st.hasMoreTokens())
			st = new StringTokenizer(br.readLine(), " ");
		return st.nextToken();
	}

	public int nextInt() throws IOException {
		return Integer.parseInt(next());
	}

	public long nextLong() throws IOException {
		return Long.parseLong(next());
	}

	public String nextLine() throws IOException {
		return br.readLine();
	}

	public boolean ready() throws IOException {
		return br.ready();
	}

	public double nextDouble() throws IOException {
		return Double.parseDouble(next());
	}

}