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 sys

lines = sys.stdin.read().split("\n")
lines.pop(-1)
n = int(lines.pop(0))
 
#Saque esta estructura de internet porque no habia otra
#implementacion nativa en python y no queria calentarme
class Node:
    """Represents an element of a set."""
    def __init__(self, id):
        self.id = id
        self.parent = self
        self.rank = 0
        self.size = 1
 
    def __repr__(self):
        return 'Node({!r})'.format(self.id)
 
 
def Find(x):
    """Returns the representative object of the set containing x."""
    if x.parent is not x:
        x.parent = Find(x.parent)
    return x.parent
 
 
def Union(x, y):
    """Combines the sets x and y belong to."""
    xroot = Find(x)
    yroot = Find(y)
 
    # x and y are already in the same set
    if xroot is yroot:
        return
 
    # x and y are not in same set, so we merge them
    if xroot.rank < yroot.rank:
        xroot, yroot = yroot, xroot  # swap xroot and yroot
 
    # merge yroot into xroot
    yroot.parent = xroot
    xroot.size += yroot.size
    if xroot.rank == yroot.rank:
        xroot.rank = xroot.rank + 1
 
nodos = [Node(i) for i in range(n+1)]
edges = []
final = []
for line in lines:
    a = nodos[int(line.split(" ")[0])]
    b = nodos[int(line.split(" ")[1])]
    if (Find(a) != Find(b)):
        Union(a, b)
    else:
        edges.append( (a,b) )
 
for i in range(2,n+1):
    if (Find(nodos[1]) != Find(nodos[i])):
        final.append( ( nodos[1],nodos[i] ) )
        Union(nodos[1],nodos[i])
        
print(len(final))
for i in range(len(final)):
    print(edges[i][0].id, end=" ")
    print(edges[i][1].id, end=" ")
    print(final[i][0].id, end=" ")
    print(final[i][1].id, end=" ")
