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 RoadsNotOnlyinBerland {
	static JP in = new JP(System.in);
	static int N,E,u,v;
	static ArrayList<ii> rem = new ArrayList();
	static ArrayList<ii> ans = new ArrayList();
	static PrintWriter out = new PrintWriter(System.out);
	public static void main(String[]args) throws Throwable{
		N =  in.j();
		DSU1 dsu = new DSU1(N);
		E = N-1;
		for(int i=0;i<E;i++){
			u = in.j(); v = in.j();
			u--;
			v--;
			if(!dsu.isSameSet(u,v)) dsu.union(u, v);
			else rem.add(new ii(u,v));

		}
		int days = rem.size();
		for(int i=0;i<N;i++){
			for(int j=0;j<N;j++){
				if(!dsu.isSameSet(i,j)){
					dsu.union(i,j);
					ans.add(new ii(i,j));
				}
			}
		}
		out.printf("%d\n",days);
		//System.err.println(days+" "+ans.size());
		for(int i=0;i<days;i++){
			int f1 = (rem.get(i).fst+1);
			int t1 = (rem.get(i).snd+1);
			int f2 = (ans.get(i).fst+1);
			int t2 = (ans.get(i).snd+1);

			out.printf("%d %d %d %d\n",f1,t1,f2,t2); 
		}
		out.flush();

	}
}

class ii{
	int fst;
	int snd;
	public ii(int fst,int snd){
		this.fst = fst;
		this.snd=snd;
	}
}


class JP {
	public BufferedReader reader;
	public StringTokenizer tokenizer;

	public JP(InputStream stream) {
		reader = new BufferedReader(new InputStreamReader(stream), 32768);
		tokenizer = null;
	}

	public String next() {
		while (tokenizer == null || !tokenizer.hasMoreTokens()) {
			try {
				tokenizer = new StringTokenizer(reader.readLine());
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
		}
		return tokenizer.nextToken();
	}

	public int j() {
		return Integer.parseInt(next());
	}


}
class DSU1 {
	int rank[];
	int parent[];
	int N;
	int sets;
	int setsize[];

	public DSU1(int N){
		this.N = N;
		sets = N;
		parent = new int[N];
		rank = new int[N];
		setsize = new int[N];
		for(int i=0;i<N;i++){ parent[i] = i; setsize[i] = 1; }

	}

	public void union(int x,int y){
		if(isSameSet(x,y)) return;
		sets--;
		int xp = getParent(x);
		int yp = getParent(y);
		if(rank[xp]>rank[yp]){ parent[yp] = xp; setsize[xp]+=setsize[yp];}
		else{
			if(rank[xp]==rank[yp]){
				rank[yp]++;
				parent[xp] = yp;
				setsize[yp]+=setsize[xp];
			}
			else
			{ parent[xp] =yp; setsize[yp] += setsize[xp]; }
		}

	}

	public int getParent(int x){
		if(x==parent[x]) return x;
		return parent[x] = getParent(parent[x]);
	}

	public boolean isSameSet(int x,int y){
		return getParent(x)==getParent(y);
	}

	public int numDisjointSets(){
		return sets;
	}
	public int getSetSize(int x){
		return setsize[getParent(x)];
	}



}
