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 
'''

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'''

# import numpy as np

parent = [0 for i in range(1001)]
rank = [0 for i in range(1001)]

def make_set(num):
    parent[num] = num
    rank[num] = 1
    
def find_set(a):
    if parent[a] == a:
        return a
    parent[a] = find_set(parent[a])
    return parent[a]
    
def union_sets(a, b):
    x = find_set(a)
    y = find_set(b)
    
    if x != y:
        if rank[x] < rank[y]:
            x, y = y, x
            
        parent[y] = x
        if rank[x] == rank[y]:
            rank[x] += 1
        
n = int(input())

for i in range(1, n+1):
    make_set(i)
    
to_remove = []    
for i in range(1, n):
    [a, b] = list(map(int, input().split()))
    # print (a, b)
    #b = int(input())
    if find_set(a) == find_set(b):
        to_remove.append((a, b))
    else:
        union_sets(a, b)
        
is_parent = [0 for i in range(1001)]
parents = []
for i in range(1, n+1):
    p = find_set(i)
    if is_parent[p] == 0:
        is_parent[p] = 1
        parents.append(p)
    
print(len(to_remove))
for i in range(len(to_remove)):
    print(to_remove[i][0], to_remove[i][1], parents[i], parents[i+1])