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.InputStreamReader;
import java.util.StringTokenizer;


public class Main {
    
    static int parent[];        // parent
    static int rank[];
    
    static void makeSet(int x) {
        parent[x] = x;
        rank[x] = 0;
    }
    
    static int find(int x) {
        
        if (parent[x] == x)
            return x;
        
        parent[x] = find(parent[x]);
        return parent[x];
    }
    
    static void union(int x, int y) {
        int xRoot = find(x);
        int yRoot = find(y);
        
        if (xRoot == yRoot)
            return;
        
        if (rank[xRoot] < rank[yRoot]) {
            parent[xRoot] = yRoot;
        } else {
            if (rank[xRoot] > rank[yRoot]) {
                parent[yRoot] = xRoot;
            } else {
                parent[yRoot] = xRoot;
                rank[xRoot]++;
            }
        }
    }
    
    public static void main(String[] args) throws Exception {
        int i,j,k;
        int x,y;
        
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st;
        
        int N = Integer.parseInt(br.readLine());
        
        parent = new int[N+1];
        rank = new int[N+1];
        
        for (i=1;i<=N;i++)
            makeSet(i);
        
        int xRoot;
        int yRoot;
        
        int T = 0;
        
        int a[] = new int[N];
        int b[] = new int[N];
        
        int M = 0;
        
        int na[] = new int[N];
        int nb[] = new int[N];
        
        for (i=1;i<N;i++) {
            st = new StringTokenizer(br.readLine());
            x = Integer.parseInt(st.nextToken());
            y = Integer.parseInt(st.nextToken());
            
            xRoot = find(x);
            yRoot = find(y);
            
            if (xRoot != yRoot) {
                union(x, y);
            } else {
                a[T] = x;
                b[T] = y;
                T++;
            }
            
        }
        
        int root = find(1);
        
        for (i=2;i<=N;i++) {
            xRoot = find(i);
            
            if (xRoot != root) {
                na[M] = xRoot;
                nb[M] = root;
                M++;
                
                union(xRoot, root);
                root = find(i);
            }
            
        }
        
        System.out.println(T);
        
        for (i=0;i<T;i++) {
            System.out.println(a[i]+" "+b[i]+" "+na[i]+" "+nb[i]);
        }
        
    }

}
