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.HashSet;
import java.util.LinkedList;

import static java.lang.Integer.*;
import static java.lang.Math.*;
@SuppressWarnings("unused")
public class D {
    static boolean [][] graph;
    static boolean [] vis;
    static int cycleStart = -1;
    static boolean [] cyclePath;
    static LinkedList<Integer> cycle = new LinkedList<Integer>();
    static HashSet<Integer> vertInCycles = new HashSet<Integer>();
    // EFFICIENT CYCLE DETECTION AND GETTING THE CYCLE
    public static void dfs(int idx, int from){
        if(vis[idx]){
            cycleStart = idx;
            cyclePath[from] = true;
            if(!cycle.contains(idx))
                cycle.add(idx);
            return;
        }
        vis[idx] = true;
        for(int i = 0 ; i < graph[idx].length ; ++i){
            if(i == idx || i == from || !graph[idx][i])
                continue;
            dfs(i, idx);
        }
        if(cycleStart != -1)
            vertInCycles.add(idx);
        if(cyclePath[idx]){
            if(idx == cycleStart)
                return;
            if(from != -1)
                cyclePath[from] = true;
            if(!cycle.contains(idx))
                cycle.add(idx);
        }
    }
    public static void main(String[] args)throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = parseInt(br.readLine());
        String [] use = null;
        graph = new boolean [n][n];
        for(int i = 0 ; i < n - 1; ++i){
            use = br.readLine().split(" ");
            int a = parseInt(use[0]);
            --a;
            int b = parseInt(use[1]);
            --b;
            graph[a][b] = graph[b][a] = true;
        }
        vis = new boolean [n];
        LinkedList<LinkedList<Integer>> allCycles = new LinkedList<LinkedList<Integer>>();
        for(int i = 0 ; i < n ; ++i){
            if(vis[i])
                continue;
            cycle = new LinkedList<Integer>();
            cycleStart = -1;
            cyclePath = new boolean [n];
            dfs(i, -1);
            if(cycleStart != -1){
                allCycles.addLast(cycle);
            }
        }
        int toConnectTo = -1;
        for(int i = 0 ; i < n ; ++i){
            if(!vertInCycles.contains(i)){
                toConnectTo = i;
                break;
            }
        }
        System.out.println(allCycles.size());
        for(int i = 0 ; i < allCycles.size() ; ++i){
            LinkedList<Integer> tmp = allCycles.get(i);
            System.out.println((tmp.get(1) + 1) + " " + (tmp.get(0) + 1)
                    + " " + (tmp.get(1) + 1) + " " + (toConnectTo + 1));
        }
        
        
    }

}
