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.*;
import java.util.*;

public class D implements Runnable{
	public static void main (String[] args) {new Thread(null, new D(), "_cf", 1 << 28).start();}

	int n;
	ArrayList<Edge>[] adj;
	boolean[] cycle, vis;
	
	public void run() {
		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		System.err.println("Go!");
		
		n = fs.nextInt();
		Edge[] es = new Edge[n-1];
		adj = new ArrayList[n];
		for(int i = 0; i < n; i++)
			adj[i] = new ArrayList<>();
		for(int i = 0; i < n-1; i++) {
			int u = fs.nextInt()-1;
			int v = fs.nextInt()-1;
			Edge e = new Edge(u, v, i);
			es[i] = e;
			adj[u].add(e);
			adj[v].add(e);
		}
		cycle = new boolean[n-1];
		vis = new boolean[n];
		for(int i = 0; i < n; i++) {
			if(vis[i]) continue;
			dfs(i, -1);
		}
		ArrayList<int[]> res = new ArrayList<>();
		DSU ds = new DSU(n);
		for(Edge e : es)
			ds.union(e.v, e.u);
		for(int i = 0; i < cycle.length; i++) if(cycle[i]) {
			int[] add = new int[4];
			Arrays.fill(add, -1);
			add[0] = es[i].u;
			add[1] = es[i].v;
			for(int j = 0; j < n; j++) {
				boolean go = ds.union(j, add[0]);
				if(go) {
					add[2] = add[0];
					add[3] = j;
					break;
				}
			}
			if(add[3] < 0) throw new RuntimeException("???");
			res.add(add);
		}
		out.println(res.size());
		for(int[] a : res) {
			for(int j = 0; j < 4; j++)
				out.printf("%d ", a[j]+1);
			out.println();
		}
		
		out.close();
	}
	
	void dfs(int u, int p) {
		vis[u] = true;
		for(Edge e : adj[u]) {
			int v = e.getAlt(u);
			if(v == p) continue;
			if(vis[v]) {
				cycle[e.id] = true;
			}
			else {
				dfs(v, u);
			}
		}
	}
	
	class DSU {
		int n, par[];
		DSU(int a) {
			n = a;
			par = new int[n];
			for(int i = 0; i < n; i++)
				par[i] = i;
		}
		int find(int u) {
			if(u == par[u]) return u;
			return par[u] = find(par[u]);
		}
		boolean union(int u, int v) {
			int pu = find(u);
			int pv = find(v);
			if(pu == pv) return false;
			par[pv] = pu;
			return true;
		}
	}
	
	class Edge {
		int u, v, id;
		Edge(int a, int b, int i) {
			u = a;
			v = b;
			id = i;
		}
		int getAlt(int a) {
			if(a == u) return v;
			return u;
		}
	}

	class FastScanner {
		public int BS = 1<<16;
		public char NC = (char)0;
		byte[] buf = new byte[BS];
		int bId = 0, size = 0;
		char c = NC;
		double num = 1;
		BufferedInputStream in;

		public FastScanner() {
			in = new BufferedInputStream(System.in, BS);
		}

		public FastScanner(String s) throws FileNotFoundException {
			in = new BufferedInputStream(new FileInputStream(new File(s)), BS);
		}

		public char nextChar(){
			while(bId==size) {
				try {
					size = in.read(buf);
				}catch(Exception e) {
					return NC;
				}                
				if(size==-1)return NC;
				bId=0;
			}
			return (char)buf[bId++];
		}

		public int nextInt() {
			return (int)nextLong();
		}

		public long nextLong() {
			num=1;
			boolean neg = false;
			if(c==NC)c=nextChar();
			for(;(c<'0' || c>'9'); c = nextChar()) {
				if(c=='-')neg=true;
			}
			long res = 0;
			for(; c>='0' && c <='9'; c=nextChar()) {
				res = (res<<3)+(res<<1)+c-'0';
				num*=10;
			}
			return neg?-res:res;
		}

		public double nextDouble() {
			double cur = nextLong();
			return c!='.' ? cur:cur+nextLong()/num;
		}

		public String next() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c>32) {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public String nextLine() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c!='\n') {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public boolean hasNext() {
			if(c>32)return true;
			while(true) {
				c=nextChar();
				if(c==NC)return false;
				else if(c>32)return true;
			}
		}
		
		public int[] nextIntArray(int n) {
			int[] res = new int[n];
			for(int i = 0; i < n; i++) res[i] = nextInt();
			return res;
		}
		
	}

}